01 - Grease a 9x13-inch baking dish with unsalted butter or nonstick spray.
02 - Arrange half of the English muffin cubes evenly in the prepared baking dish, then scatter half of the diced Canadian bacon over the top. Repeat layering with remaining English muffins and Canadian bacon.
03 - In a large mixing bowl, whisk together eggs, whole milk, heavy cream, Dijon mustard, salt, black pepper, paprika, and melted butter until well combined.
04 - Pour the egg mixture evenly over the layered muffins and Canadian bacon, pressing gently to ensure all pieces are at least partially submerged.
05 - Cover the dish and refrigerate for at least 2 hours or preferably overnight to allow the custard to absorb.
06 - Preheat oven to 350 degrees Fahrenheit. Remove the assembled casserole from the refrigerator while the oven heats.
07 - Bake the casserole uncovered for 40 to 45 minutes, or until puffed, golden brown, and set in the center. Allow to rest for 10 minutes out of the oven.
08 - In a heatproof mixing bowl, whisk together egg yolks and lemon juice until combined and slightly thickened. Place the bowl over a saucepan of simmering water, ensuring the bottom does not touch the water. While whisking constantly, slowly stream in melted butter until sauce thickens. Season with a pinch of cayenne and salt to taste. Remove from heat.
09 - Slice the baked casserole, distribute onto plates, and drizzle each portion generously with hollandaise sauce. Serve immediately.