01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking dish with butter or nonstick 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 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 incorporated and smooth.
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 refrigerated overnight, remove the casserole from the refrigerator and let it stand 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, set a heatproof bowl over a saucepan of barely simmering water to create a double boiler. Whisk the egg yolks and lemon juice together in the bowl until pale and combined. Slowly drizzle in the melted butter in a thin steady stream while whisking constantly until the sauce thickens, approximately 5 minutes. Stir in the Dijon mustard, season with salt and pepper, and remove from heat.
08 - Allow the baked casserole to rest for 10 minutes before slicing into portions. Serve warm, drizzled generously with the fresh hollandaise sauce.