01 - Generously grease a 9x13-inch baking dish with butter or cooking spray. Distribute half the English muffin pieces evenly across the bottom. Layer with half the diced Canadian bacon. Repeat with remaining muffin pieces and bacon to create two complete layers.
02 - In a large mixing bowl, whisk together 8 eggs, 2 cups whole milk, 1 cup heavy cream, Dijon mustard, kosher salt, black pepper, and paprika until completely smooth and well combined. Pour the mixture evenly over the layered casserole, ensuring all bread pieces are saturated. Gently press down with a spatula to help the bread absorb the liquid.
03 - Cover the baking dish tightly with aluminum foil. Refrigerate for at least 8 hours or overnight to allow the flavors to meld and the bread to fully absorb the custard.
04 - Remove the casserole from the refrigerator and let it stand at room temperature while the oven preheats to 375°F. This allows for even cooking.
05 - Bake covered with foil for 30 minutes. Remove the foil and continue baking for an additional 15 minutes until the casserole is puffed, golden brown, and the center is completely set.
06 - While the casserole bakes, set a heatproof bowl over a saucepan of simmering water, ensuring the bowl does not touch the water. Whisk egg yolks and lemon juice vigorously until thickened, approximately 2 minutes. Slowly drizzle in the hot melted butter while whisking constantly until the sauce is smooth and emulsified. Season with cayenne and salt to taste. Keep warm until serving.
07 - Allow the baked casserole to rest for 10 minutes before slicing into portions. Drizzle each serving generously with warm Hollandaise sauce. Garnish with fresh chives and a light sprinkle of paprika if desired.