01 - Grease a 9x13-inch baking dish thoroughly with butter or cooking spray to prevent sticking.
02 - 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 with remaining muffins and bacon to create two complete layers.
03 - In a large bowl, whisk together the eggs, whole milk, salt, black pepper, Dijon mustard, and paprika until fully combined and smooth.
04 - Pour the egg mixture evenly over the layered muffins and bacon. Gently press down with a spatula or the back of a spoon to ensure the bread absorbs the liquid.
05 - Cover the dish with plastic wrap or aluminum foil. Refrigerate for at least 1 hour, or overnight for optimal flavor and texture development.
06 - Preheat oven to 375°F.
07 - Remove the casserole from the refrigerator while the oven heats. Bake uncovered for 40 to 45 minutes until the center is set and the top is golden brown.
08 - Melt the butter in a small saucepan and keep warm. In a heatproof bowl set over a pot of gently simmering water, whisk together egg yolks, lemon juice, Dijon mustard, salt, and cayenne. Slowly drizzle in the melted butter while whisking constantly until the sauce thickens and becomes smooth. Remove from heat immediately.
09 - Let the baked casserole rest for 10 minutes before slicing. Cut into 8 equal portions and serve warm, generously drizzled with the hollandaise sauce. Garnish with fresh chives or parsley if desired.