01 - Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Scatter half of the English muffin pieces evenly across the bottom of the dish. Top with half of the diced Canadian bacon. Repeat with the remaining muffins and bacon to form a second layer.
03 - In a large bowl, whisk together the eggs, whole milk, Dijon mustard, salt, and black pepper until fully combined. Pour the mixture evenly over the layered muffins and bacon. Cover tightly with plastic wrap and refrigerate for at least 2 hours, or preferably overnight to allow the flavors to meld.
04 - When ready to bake, preheat the oven to 350°F. Remove the casserole from the refrigerator and let it stand at room temperature while the oven comes to temperature.
05 - Bake uncovered for 40 to 45 minutes, or until the center is set and the top is golden brown. If the top begins to brown too quickly, loosely tent with aluminum foil.
06 - While the casserole bakes, set a heatproof bowl over a saucepan of barely simmering water to create a double boiler. Add the egg yolks, lemon juice, Dijon mustard, and cayenne pepper. Whisk constantly for about 2 minutes until the mixture slightly thickens. Slowly drizzle in the melted butter while whisking continuously until the sauce is thick, glossy, and emulsified. Season with salt to taste.
07 - Remove the casserole from the oven and drizzle generously with the warm hollandaise sauce. Garnish with finely chopped fresh chives and a light sprinkle of paprika. Serve immediately.