01 - Lightly grease a 9x13-inch baking dish to prevent sticking.
02 - Scatter half of the English muffin pieces evenly across the bottom of the prepared dish. Layer half of the diced Canadian bacon over the muffins. Repeat the process with the remaining muffin pieces and bacon.
03 - In a large mixing bowl, vigorously whisk together the eggs, whole milk, Dijon mustard, salt, and black pepper until fully incorporated. Pour the custard evenly over the assembled muffin and bacon layers. Cover tightly with plastic wrap and refrigerate for a minimum of 2 hours, ideally overnight.
04 - Preheat the oven to 350°F. Remove the casserole from the refrigerator and allow it to rest at room temperature while the oven comes to temperature.
05 - Bake the casserole uncovered for 40 to 45 minutes, or until the center is fully set and the top achieves a golden-brown color. If the surface begins to brown prematurely, tent it loosely with aluminum foil.
06 - While the casserole bakes, set a heatproof bowl over a saucepan of barely simmering water to create a double boiler. Combine the egg yolks, lemon juice, Dijon mustard, and cayenne pepper in the bowl. Whisk constantly for roughly 2 minutes until the mixture slightly thickens. Slowly stream in the melted butter while continuing to whisk until the sauce becomes thick and glossy. Adjust the seasoning with salt as needed.
07 - Remove the baked casserole from the oven. Drizzle the warm hollandaise generously over the top. Garnish with freshly chopped chives and a light dusting of paprika before serving immediately.