01 - Grease a 9x13-inch baking dish thoroughly with butter or cooking spray. Distribute half the English muffin pieces evenly across the bottom. Cover with half the Canadian bacon. Repeat with remaining muffins and bacon to create two complete layers.
02 - Whisk together eggs, milk, heavy cream, Dijon mustard, salt, pepper, and paprika in a large bowl until fully incorporated. Pour the mixture evenly over the layered casserole, ensuring all bread pieces are saturated. Gently press down with a spatula to submerge any dry pieces.
03 - Cover the baking dish tightly with aluminum foil. Refrigerate for at least 8 hours or overnight to allow the bread to fully absorb the custard and flavors to meld.
04 - Preheat your oven to 375°F. Remove the casserole from the refrigerator and let it stand at room temperature while the oven heats, approximately 20-30 minutes, to ensure even cooking.
05 - Bake covered with foil for 30 minutes. Remove the foil and continue baking for 15 minutes longer, until the casserole is puffed, golden brown, and the center is completely set when tested with a knife.
06 - While the casserole finishes baking, set a heatproof bowl over a saucepan of simmering water, ensuring the bowl does not touch the water. Whisk egg yolks and lemon juice constantly until thickened, about 2 minutes. Slowly drizzle in the hot melted butter while whisking continuously until emulsified and creamy. Season with cayenne and salt to taste. Keep warm until serving.
07 - Allow the casserole to rest for 10 minutes before slicing into portions. Drizzle each serving generously with warm Hollandaise sauce. Sprinkle with fresh chives and a dusting of paprika if desired.