01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - Arrange half of the English muffin pieces in the prepared dish. Cover with half of the Canadian bacon. Repeat layers with remaining muffins and bacon.
03 - Whisk together 8 large eggs, 2 cups milk, 1 tsp Dijon mustard, ½ tsp salt, ¼ tsp pepper, ¼ tsp paprika, and 2 tbsp melted butter in a large bowl until smooth.
04 - Pour the egg mixture evenly over the casserole. Gently press down on the bread to ensure it absorbs the liquid.
05 - Cover the dish with aluminum foil and refrigerate for at least 1 hour, or overnight, to allow flavors to meld.
06 - Remove from refrigerator 15 minutes before baking. Bake covered at 350°F for 30 minutes.
07 - Remove foil and bake for an additional 15 minutes until the eggs are set and the top is golden brown.
08 - While casserole bakes, whisk 4 egg yolks and 1 tbsp lemon juice in a heatproof bowl set over a pan of simmering water.
09 - Slowly drizzle in ½ cup melted butter while whisking constantly until sauce thickens. Stir in ½ tsp Dijon mustard and season with salt and pepper.
10 - Let the baked casserole rest for 10 minutes before slicing. Serve warm drizzled with the warm hollandaise sauce.