Turkey Tetrazzini Mushrooms Peas Casserole

Turkey Tetrazzini arrived in my kitchen on a gray November afternoon when I had a container of leftover roasted turkey and absolutely no inspiration. I stood at the pantry looking at what I had—pasta, mushrooms, cream—and suddenly remembered my grandmother's version, which she'd make the day after Thanksgiving. The result was pure comfort: creamy, deeply savory, with those golden cheese-crusted edges that you immediately want to scrape into your bowl.

I made this for a potluck once, not realizing how deeply people would connect with it. A friend who'd been having a rough week came back for seconds and just sat there quietly with her fork, and I understood then that the best recipes aren't the most impressive ones—they're the ones that make you feel held.

Ingredients

• Spaghetti or fettuccine (350 g / 12 oz): The thinner pasta works best here because it lets the sauce coat every strand without weighing the dish down—cook it just shy of al dente so it firms up slightly during baking.
• Cooked turkey breast (2 cups), shredded or cubed: Leftover roasted turkey is ideal, but rotisserie chicken works beautifully too and honestly might be easier to source.
• Cremini or white mushrooms (250 g / 9 oz), sliced: These earthier cremini mushrooms give more depth than button mushrooms, and slicing them thin means they'll release their moisture faster and develop that golden color.
• Frozen peas (1 cup), thawed: Frozen peas are your friend here—they're picked at peak ripeness and stay bright and sweet, much more reliable than fresh ones sitting in your crisper drawer.
• Small onion, finely chopped: Keeps the casserole textured without overwhelming it; the fine chop means it softens completely into the sauce.
• Garlic cloves (2), minced: Just enough to whisper in the background rather than announce itself—fresh is essential here.
• Unsalted butter (4 tbsp for sauce, 2 tbsp for mushrooms, 2 tbsp melted for topping): I use unsalted so you're in control of the salt level, and because the Parmesan is already generous with it.
• All-purpose flour (4 tbsp): This makes the roux that thickens everything without any lumps if you whisk constantly.
• Whole milk (2 cups): Full-fat milk makes the sauce actually taste like milk, not like water with a vague memory of dairy.
• Chicken or turkey broth (1 cup): Adds savory depth without making the dish heavy.
• Heavy cream (120 ml / ½ cup): This is what makes it silky and luxurious—don't skip it or try to substitute.
• Black pepper, salt, ground nutmeg: Nutmeg in cream sauces is the secret nobody talks about—it adds warmth without tasting spiced.
• Grated Parmesan cheese (1 cup total), divided: Half goes into the sauce where it melts into pure umami, and half goes on top to get crispy and golden.
• Shredded mozzarella (1 cup): This melts into stretchy pockets throughout the casserole rather than staying on top.
• Panko breadcrumbs (½ cup): The larger crumbs stay crunchy even after baking and absorb the butter perfectly.
• Fresh parsley (2 tbsp, optional): A handful scattered on top right at the end brings a little brightness to an otherwise rich dish.

Instructions

Get your oven ready and prep your dish:

Preheat to 190°C (375°F) and grease your 23x33 cm (9x13 inch) casserole dish—I use a little butter on a paper towel so it gets into the corners.

Cook the pasta until just al dente:

In salted boiling water, cook it one minute short of what the package says. It'll finish cooking in the oven and you don't want mushy pasta by the time you eat it.

Sauté the mushrooms and aromatics:

In a large skillet over medium heat, melt 2 tbsp butter and add your chopped onion—let it soften for about 2 minutes until it's turning translucent. Add the sliced mushrooms and cook until they're golden and you can see the liquid they released has mostly evaporated, about 6 minutes. Stir in the garlic for just 1 more minute so it softens but doesn't brown.

Make a silky sauce:

In a separate saucepan, melt the remaining 2 tbsp butter over medium heat. Whisk in the flour and cook for 1 minute to toast it slightly. Slowly pour in the milk and broth while whisking constantly to avoid lumps. Bring it to a gentle simmer and cook for 3-4 minutes until it thickens enough to coat the back of a spoon. Stir in the cream, salt, pepper, nutmeg, and ½ cup of the Parmesan until the cheese melts completely and everything tastes balanced.

Bring everything together:

In a large bowl, combine the cooked pasta, shredded turkey, mushroom mixture, thawed peas, mozzarella, and the cream sauce. Toss gently but thoroughly so everything is evenly coated and the turkey isn't clumped in one spot.

Transfer to the casserole:

Pour the mixture into your prepared dish and spread it so it's relatively even. Sprinkle the remaining ½ cup of Parmesan over the top—this will get golden and crispy as it bakes.

Make the crunchy topping:

Mix the panko breadcrumbs with the 2 tbsp melted butter until every crumb is coated slightly, then sprinkle this evenly over the top. This is what catches the heat and turns golden and crunchy.

Bake until golden and bubbly:

Bake for 25-30 minutes—you're looking for the edges to bubble up around the sides and the top to turn a deep golden brown. Let it rest for 10 minutes after coming out of the oven so it sets slightly and is easier to serve. Scatter parsley over the top if you want a little green brightness.

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There's a moment right when this comes out of the oven, when the cheese on top is still bubbling at the edges and the whole kitchen smells like butter and mushrooms and cream, that feels almost ceremonial. You know the meal is going to be good—no question about it.

Timing and Make-Ahead Options

You can assemble the entire casserole the night before, cover it with plastic wrap, and bake it the next day—just add 5-10 minutes to the baking time since it starts cold. I've also frozen it after baking and reheated it gently in a low oven, which keeps the texture better than you'd expect.

Variations That Actually Work

Rotisserie chicken is genuinely better sometimes because it has more flavor than leftover turkey, and it's always available. You can add a splash of dry white wine to the mushroom pan to deglaze it and capture all those browned bits. For a lighter version, use half-and-half instead of heavy cream, though the sauce won't be quite as luxurious.

Storage and Serving

This casserole keeps well in the refrigerator for up to 4 days and reheats beautifully—just cover it loosely with foil so the top doesn't dry out. Serve it with something bright alongside: a crisp green salad with a sharp vinaigrette, or steamed green beans tossed with lemon and garlic to cut through the richness.

• Leftover turkey from any roasted poultry works here, not just Thanksgiving birds.
• The casserole is forgiving if you need to swap mushroom types or use what's in your produce drawer.
• Make extra because people will ask for seconds, and the leftovers actually taste better the next day.

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This is the kind of dish that quietly becomes a favorite—not flashy, but deeply satisfying in the way that only something with butter and cream and good ingredients can be. Make it once and you'll find yourself wanting it again.

Your Recipe Questions

→ Can I substitute turkey with chicken?

Yes, rotisserie chicken works beautifully as a substitute for the turkey breast in this dish.

→ Which pasta shape is best?

Spaghetti or fettuccine are traditional choices, but any long pasta will hold the creamy sauce well.

→ Can I make this gluten-free?

Absolutely, simply swap the regular pasta and all-purpose flour for your favorite gluten-free alternatives.

→ How should I store leftovers?

Store cooled leftovers in an airtight container in the refrigerator for up to three days.

→ Can I add wine to the sauce?

Adding a splash of dry white wine to the mushrooms while sautéing adds depth to the flavor profile.