Lightly grease a 9x13 inch baking dish. Evenly spread the torn English muffin pieces in the bottom of the dish. Distribute the diced Canadian bacon over the muffins. Sprinkle 3/4 cup of the shredded Gruyere cheese evenly over the bacon layer.

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, Dijon mustard, salt, black pepper, and smoked paprika until well combined and slightly frothy.

Carefully pour the egg custard mixture over the English muffins, bacon, and cheese in the baking dish, ensuring all pieces are moistened. Gently press down any floating muffin pieces to submerge them.

Cover the baking dish tightly with plastic wrap and refrigerate for at least 8 hours, or preferably overnight (up to 24 hours).

The next morning, preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator and let it sit at room temperature for about 20-30 minutes while the oven preheats. Remove the plastic wrap.

Bake the casserole, covered with aluminum foil, for 30 minutes.

Remove the foil, sprinkle the remaining 1/4 cup of Gruyere cheese over the top, and continue to bake uncovered for another 25-30 minutes, or until the casserole is set in the center, golden brown, and puffed. An inserted knife should come out clean.

Once baked, remove the casserole from the oven and let it rest for 10-15 minutes before serving. This allows it to set fully.

While the casserole is resting, prepare the hollandaise. In a blender, combine the egg yolks, lemon juice, salt, and cayenne pepper (if using). Blend on high for about 30 seconds until pale and frothy.

With the blender running on low speed, slowly and steadily drizzle the hot melted butter into the egg yolk mixture. Continue blending until the sauce is thick, creamy, and emulsified, about 1-2 minutes. Taste and adjust seasoning if needed. Keep warm until serving.

Cut the casserole into squares. Serve each portion with a generous spoonful of warm Hollandaise sauce. Garnish with fresh chopped chives and optional parsley.