01 - Lightly grease a 9x13-inch baking dish. Scatter half of the English muffin pieces into the dish and top with half the Canadian bacon. Repeat layers with remaining muffins and bacon.
02 - Whisk together 8 large eggs, 2 cups whole milk, 1 teaspoon Dijon mustard, 1/2 teaspoon salt, and 1/4 teaspoon black pepper in a large bowl. Pour the mixture evenly over the muffin and bacon layers.
03 - Cover the dish with plastic wrap and refrigerate for at least 2 hours, or preferably overnight, to allow the bread to absorb the liquid.
04 - Remove the casserole from the refrigerator and let it stand at room temperature while the oven preheats to 350°F (180°C).
05 - 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 foil.
06 - Set a heatproof bowl over a saucepan of barely simmering water. Whisk 3 egg yolks, 1 tablespoon lemon juice, 1/2 teaspoon Dijon mustard, and cayenne constantly until thickened. Slowly drizzle in 1/2 cup melted butter while whisking until glossy. Season with salt.
07 - Drizzle the warm hollandaise sauce over the baked casserole. Garnish with fresh chives and paprika if desired. Serve immediately.