01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking dish with butter or non-stick 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 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 fully combined and smooth.
04 - Pour the egg custard mixture evenly over the layered muffins and bacon. Gently press down on the bread pieces to ensure they absorb the liquid. Cover tightly with aluminum foil and refrigerate for at least 1 hour, or overnight for optimal texture.
05 - If refrigerated overnight, remove the casserole from the refrigerator and let it sit at room temperature while the oven preheats to 350°F.
06 - Bake covered with foil for 30 minutes. Remove the foil and continue baking for an additional 15 minutes, until the center is set and the top is lightly golden.
07 - While the casserole bakes, place egg yolks and lemon juice in a heatproof bowl set over a saucepan of barely simmering water. Whisk constantly until the mixture begins to thicken. Slowly drizzle in the melted butter while whisking continuously until the sauce is thick and creamy, about 5 minutes. Stir in the Dijon mustard, season with salt and pepper, and remove from heat.
08 - Remove the casserole from the oven and let it rest for 10 minutes. Cut into portions and serve warm, generously drizzled with the fresh hollandaise sauce.