Grease a 9x13 inch baking dish. Evenly spread the cubed English muffins in the bottom of the prepared dish.

Sprinkle the diced Canadian bacon (or ham) over the English muffins.

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, Dijon mustard, salt, pepper, and paprika (if using) until well combined.

Pour the egg mixture evenly over the English muffins and Canadian bacon. Gently press down with a spatula to ensure the muffins absorb the liquid.

Evenly sprinkle the shredded Swiss or Gruyere cheese over the top of the casserole.

At this point, you can cover the casserole with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, for best flavor and texture. Or, bake immediately.

Preheat your oven to 375°F (190°C).

If baking from cold (refrigerated), remove the casserole from the fridge about 30 minutes before baking. Bake uncovered for 45-50 minutes, or until the casserole is set in the center, golden brown on top, and a knife inserted comes out clean. If the top browns too quickly, you can loosely tent it with foil.

Let the casserole rest for 5-10 minutes before serving.

While the casserole is baking, melt the butter in a small saucepan until it's very hot and foamy, but not browned.

In a blender or food processor, combine the egg yolks, lemon juice, Dijon mustard (if using), salt, and cayenne pepper. Blend on high for about 30 seconds until light in color and frothy.

With the blender running on low speed, slowly and steadily pour the hot melted butter in a thin stream through the lid opening. Continue blending until the sauce is thick, creamy, and emulsified. This should take about 1-2 minutes.

Taste and adjust seasoning if needed. If the sauce is too thick, add a teaspoon of hot water and blend briefly. Serve immediately or keep warm in a thermos for up to an hour.

Spoon generous amounts of the warm hollandaise sauce over individual servings of the casserole. Garnish with fresh chopped chives, if desired, and serve immediately.