01 - Lightly grease a 9x13-inch baking dish with butter or nonstick cooking 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 mixing bowl, whisk together the eggs, whole milk, Dijon mustard, salt, and black pepper until fully combined. Pour the custard 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 bread to absorb the custard.
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, until the center is firmly set and the top is golden brown. If the surface browns too quickly, loosely tent with aluminum foil during the final 10 minutes.
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 in a thin stream while whisking continuously until the sauce becomes thick, glossy, and emulsified. Season with salt to taste.
07 - Remove the casserole from the oven and let it rest for 5 minutes. Drizzle generously with warm hollandaise sauce, then garnish with chopped fresh chives and a light dusting of paprika. Serve immediately while warm.