01 - Lightly grease a 9x13-inch baking dish with butter or cooking spray.
02 - Scatter half of the English muffin pieces into the prepared dish. Top with half of the diced Canadian bacon. Repeat the layering with the remaining muffins and bacon.
03 - In a large bowl, whisk together the 8 large eggs, whole milk, 1 teaspoon of Dijon mustard, 1/2 teaspoon of salt, and 1/4 teaspoon of black pepper until fully blended.
04 - Pour the egg mixture evenly over the muffins and bacon. Cover the dish with plastic wrap and refrigerate for at least 2 hours, or preferably overnight, to allow the bread to soak.
05 - Remove the casserole from the refrigerator and let it stand at room temperature while the oven preheats to 350°F (180°C).
06 - Bake uncovered for 40 to 45 minutes, until the center is set and the top is golden brown. If the top browns too quickly, cover loosely with aluminum foil.
07 - While the casserole bakes, set a heatproof bowl over a saucepan of barely simmering water to create a double boiler. Whisk the 3 egg yolks, 1 tablespoon lemon juice, 1/2 teaspoon Dijon mustard, and cayenne pepper together. Whisk constantly for about 2 minutes until slightly thickened. Slowly drizzle in the melted butter while whisking until the sauce is thick and glossy. Season with salt to taste.
08 - Drizzle the warm hollandaise sauce over the baked casserole. Garnish with chopped fresh chives and a sprinkle of paprika. Serve immediately.