01 - Lightly grease a 9x13-inch baking dish. Scatter half of the English muffin pieces evenly across the bottom, followed by half of the diced Canadian bacon. Repeat the layers with the remaining muffins and bacon.
02 - In a large mixing bowl, thoroughly whisk together the eggs, whole milk, Dijon mustard, salt, and black pepper. Pour this custard mixture evenly over the layered muffins and bacon. Cover tightly with plastic wrap and refrigerate for at least 2 hours, preferably overnight.
03 - Preheat your oven to 350°F. Remove the casserole from the refrigerator and let it sit at room temperature while the oven comes to temperature. Bake uncovered for 40 to 45 minutes until the center is fully set and the top is golden brown, using foil if the top browns too quickly.
04 - Set a heatproof bowl over a saucepan of barely simmering water to create a double boiler. Vigorously whisk the egg yolks, lemon juice, Dijon mustard, and cayenne pepper until slightly thickened, about 2 minutes. Gradually drizzle in the melted butter while whisking continuously until the sauce is thick and glossy. Season with salt to taste.
05 - Remove the baked casserole from the oven. Generously drizzle the warm hollandaise sauce over the top. Garnish with fresh chives and a dusting of paprika before serving immediately.