01 - Lightly grease a 9x13-inch baking dish.
02 - Scatter half of the English muffin pieces in the dish and top with half the Canadian bacon. Repeat with the remaining muffins and bacon.
03 - In a large bowl, whisk together eggs, milk, Dijon mustard, salt, and black pepper.
04 - Pour the egg mixture evenly over the muffin and bacon layers. Cover with plastic wrap and refrigerate for at least 2 hours, preferably overnight.
05 - When ready to bake, preheat the oven to 350°F. Remove the casserole from the refrigerator and let stand at room temperature while the oven preheats.
06 - Bake uncovered for 40–45 minutes, or until the center is set and the top is golden. If the top begins to brown too quickly, cover loosely with foil.
07 - While the casserole bakes, set a heatproof bowl over a saucepan of barely simmering water. Add egg yolks, lemon juice, Dijon, and cayenne. Whisk constantly until slightly thickened. Slowly drizzle in melted butter while whisking until thick and glossy. Season with salt.
08 - Remove casserole from the oven. Drizzle with warm hollandaise sauce, garnish with chives and paprika if desired, and serve immediately.