01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or non-stick spray.
02 - Evenly distribute half of the English muffin pieces across the bottom of the prepared dish. Scatter half of the chopped Canadian bacon over the muffin layer. 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 fully combined and smooth.
04 - Pour the egg custard mixture evenly over the layered muffins and bacon. Gently press down on the top layer to ensure all pieces are submerged and absorbing the liquid.
05 - Cover the dish tightly with aluminum foil and refrigerate for at least 1 hour, or preferably overnight, to allow the muffins to fully absorb the custard.
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. Remove the foil and continue baking for an additional 15 minutes, or until the center is set and the top is lightly golden.
08 - While the casserole bakes, whisk the egg yolks and lemon juice together in a heatproof bowl. Set the bowl over a saucepan of barely simmering water to create a double boiler, whisking constantly.
09 - Slowly drizzle in the melted butter while whisking continuously until the sauce thickens and becomes velvety, about 5 minutes. Stir in the Dijon mustard, season with salt and pepper, then remove from heat.
10 - Remove the casserole from the oven and let it rest for 10 minutes. Slice into portions and serve warm, generously drizzled with the freshly made hollandaise sauce.