Prepare the casserole:
Spray a 13×9-inch baking dish with nonstick spray. Cut Canadian bacon into 1-inch pieces and set aside. In a large mixing bowl, whisk together the eggs, milk, onion powder, kosher salt, and black pepper.

Toast the muffins:
Turn on your oven’s broiler. Melt butter in a small saucepan. Split the English muffins and brush the cut sides with melted butter. Broil for 4–5 minutes until golden brown, watching closely to avoid burning.

Assemble:
Cut toasted muffins into 1-inch pieces. Layer half in the bottom of the baking dish, top with half of the Canadian bacon. Repeat with remaining muffins and bacon. Pour the egg mixture evenly over the top. Cover with plastic wrap and refrigerate overnight.

Bake:
Preheat oven to 375°F. Remove plastic wrap, sprinkle the top of the casserole with paprika, and loosely cover with foil. Bake for 30 minutes, remove the foil, and continue baking for 15–20 minutes, or until the eggs are fully set. Let rest 10–15 minutes before serving.

Make the hollandaise sauce:
While the casserole bakes, prepare the hollandaise using a double boiler. Fill a saucepan with 1–2 inches of water and bring to a gentle simmer. In a heatproof bowl, whisk together egg yolks, milk, and lemon juice. Place over the simmering water and whisk constantly until the mixture thickens slightly, 3–5 minutes. Remove the bowl from the heat and slowly whisk in the warm melted butter. If needed, return the bowl to the double boiler briefly to help the sauce finish thickening. Season with salt and cayenne (if using).
Prefer a shortcut? See note #1 for an easy hollandaise sauce mix option.

Serve:
Spoon warm hollandaise sauce over each portion and garnish with minced chives, if desired.