01 - Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or nonstick 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, smoked paprika, and melted butter until smooth and well combined.
04 - Pour the egg custard evenly over the layered muffins and bacon. Gently press down on the pieces to ensure everything is submerged and soaking up the mixture.
05 - Cover the dish tightly with aluminum foil and refrigerate for at least 1 hour, or leave overnight for the best texture and flavor absorption.
06 - If chilled overnight, remove the casserole from the refrigerator and let it sit at room temperature while the oven preheats.
07 - 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.
08 - 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 yolks begin to thicken and pale in color.
09 - Slowly drizzle in the melted butter while whisking continuously until the sauce is thick and velvety, about 5 minutes. Stir in the Dijon mustard, season with salt and pepper, and remove from heat.
10 - Let the finished casserole rest for 10 minutes before slicing. Serve warm portions generously drizzled with the fresh hollandaise sauce.