01 - Preheat oven to 350°F. Grease a 9×13-inch baking dish with butter or nonstick cooking spray.
02 - Arrange half of the English muffin pieces evenly across the bottom of the prepared dish. Scatter half of the chopped Canadian bacon over the muffins. Repeat with the remaining muffins 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 fully incorporated.
04 - Pour the egg custard evenly over the layered muffins and bacon. Gently press down on the top layer so the bread absorbs the liquid. Cover tightly with aluminum foil and refrigerate for at least 1 hour or up to overnight for optimal texture.
05 - If chilled overnight, let the casserole sit at room temperature while the oven preheats. Bake covered with foil for 30 minutes, then remove the foil and continue baking for 15 minutes until the center is set and the top is lightly golden.
06 - 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 blended. Continue whisking constantly while slowly drizzling in the melted butter. Keep whisking until the sauce thickens to a creamy consistency, approximately 5 minutes. Stir in the Dijon mustard, season with salt and pepper, and remove from heat.
07 - Remove the casserole from the oven and let it rest for 10 minutes. Cut into portions and serve warm, drizzled generously with the freshly made hollandaise sauce.