01 - Grease a 9x13-inch baking dish. Scatter half the English muffin pieces evenly in the dish. Top with half the Canadian bacon. Repeat layers with remaining muffins and bacon.
02 - In a large bowl, whisk together eggs, milk, heavy cream, Dijon mustard, salt, pepper, and paprika until well combined. Pour evenly over the muffin and bacon layers. Gently press down so all bread is moistened.
03 - Cover tightly with foil and refrigerate overnight for at least 8 hours to allow flavors to meld and bread to absorb the custard.
04 - Preheat oven to 375°F. Remove casserole from refrigerator and let stand at room temperature while oven preheats.
05 - Bake, covered, for 30 minutes. Remove foil and bake an additional 15 minutes, until puffed and golden and center is set.
06 - While casserole bakes, make the Hollandaise: In a heatproof bowl set over (not in) simmering water, whisk egg yolks and lemon juice until thickened, about 2 minutes. Slowly drizzle in hot melted butter, whisking constantly, until smooth and thick. Stir in cayenne and salt to taste. Keep warm.
07 - Let casserole cool 10 minutes before slicing. Drizzle portions with warm Hollandaise sauce. Garnish with chives and a sprinkle of paprika, if desired.