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