01 - Preheat the oven to 350°F. Generously grease a 9×13-inch baking dish with butter or nonstick cooking spray.
02 - Arrange half of the English muffin pieces in an even layer across the bottom of the prepared dish. Scatter half of the chopped Canadian bacon over the muffins. Repeat with the remaining muffin pieces and bacon to form a second layer.
03 - In a large mixing bowl, whisk together the eggs, whole milk, Dijon mustard, salt, black pepper, paprika, and melted butter until smooth and thoroughly combined.
04 - Pour the egg custard mixture evenly over the layered muffins and bacon. Gently press down on the top layer to ensure the bread absorbs the liquid. Cover tightly with aluminum foil.
05 - Refrigerate the assembled casserole for at least 1 hour, or leave overnight for optimal texture and flavor absorption.
06 - If refrigerated overnight, remove the casserole from the refrigerator and let it sit at room temperature while the oven preheats to 350°F.
07 - Bake covered with foil for 30 minutes. Carefully remove the foil and continue baking for an additional 15 minutes, until the center is set and the top is lightly golden.
08 - While the casserole bakes, set up a double boiler by placing a heatproof bowl over a saucepan of barely simmering water. Whisk the egg yolks and lemon juice together in the bowl until smooth and combined.
09 - Slowly drizzle the melted butter into the yolk mixture in a thin, steady stream while whisking constantly. Continue whisking until the sauce thickens and becomes creamy, approximately 5 minutes. Stir in the Dijon mustard, season with salt and pepper to taste, and remove from heat.
10 - Remove the casserole from the oven and let it rest for 10 minutes. Cut into portions and serve warm, drizzled generously with the freshly prepared hollandaise sauce.