01 - Generously grease a 9x13-inch baking dish with butter or cooking spray. Scatter half of the English muffin pieces evenly across the bottom of the dish. Distribute half of the diced Canadian bacon over the muffins. Repeat layers with remaining muffins and bacon.
02 - Crack 8 large eggs into a large mixing bowl. Add whole milk, salt, black pepper, Dijon mustard, and paprika. Whisk vigorously until fully combined and slightly frothy. Pour the egg mixture evenly over the layered muffins and bacon, ensuring all pieces are coated. Press down gently with a spatula to help the bread absorb the liquid.
03 - Cover the baking dish tightly with aluminum foil or plastic wrap. Refrigerate for at least 1 hour, though overnight chilling produces the best texture and flavor development.
04 - Remove the casserole from the refrigerator while the oven preheates. Set oven temperature to 375°F and allow it to fully heat before baking.
05 - Bake the uncovered casserole for 40 to 45 minutes. The center should be set and firm when gently shaken, while the top achieves a deep golden-brown color. If the top browns too quickly, tent loosely with foil during the final minutes.
06 - Melt butter in a small saucepan and keep warm. Place egg yolks, lemon juice, Dijon mustard, salt, and cayenne in a heatproof bowl set over a pot of gently simmering water. Whisk constantly until the yolks thicken and pale in color. Slowly drizzle the warm butter into the yolks while whisking continuously until the sauce emulsifies, thickens, and becomes smooth. Remove from heat immediately.
07 - Let the baked casserole rest for 10 minutes before slicing. Cut into 8 equal portions and transfer to serving plates. Drizzle each serving generously with warm hollandaise sauce. Garnish with chopped fresh chives or parsley and additional freshly ground black pepper if desired.