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 prepared dish. Top with half of the diced Canadian bacon. Repeat with the remaining muffin pieces and bacon to create 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 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 muffins to absorb the custard.
04 - Preheat the oven to 350°F. Remove the casserole from the refrigerator and let it stand at room temperature while the oven heats up, about 20 minutes.
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, loosely tent with aluminum foil during the last 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 while whisking continuously until the sauce becomes thick, glossy, and emulsified. Season with salt to taste and keep warm.
07 - Remove the casserole from the oven and let it rest for 5 minutes. Drizzle generously with warm hollandaise sauce. Garnish with finely chopped fresh chives and a light sprinkle of paprika. Serve immediately while warm.