Split English muffins and chop into one-inch pieces. Spread the pieces on an ungreased baking sheet and broil for 1 to 2 minutes, or until bread is toasty on top. Use a spatula to flip the bread over, then broil for another minute or two.
Place half of the Canadian bacon in a greased 9x13-inch baking dish; top with broiled English muffins, then the remaining bacon. In a large bowl, whisk the eggs, milk, onion powder, and salt and pepper to taste; pour evenly over the top. Cover and refrigerate overnight.
Remove from refrigerator 30 minutes before baking and preheat oven to 375°. Sprinkle with paprika. Cover and bake for 35 minutes. Uncover; bake 10 to 15 minutes longer, or until center is set.
Meanwhile, make the sauce: In a double boiler or metal bowl over simmering water, whisk the egg yolks, cream, lemon juice, and mustard constantly until mixture reaches 160° or is thick enough to coat the back of a spoon. Reduce heat to low. Slowly drizzle in warm melted butter, whisking constantly. (You can also use the blender method to make this hollandaise sauce. Use the ingredients listed in this recipe; add the cream to the egg yolks.)
Serve sauce immediately with casserole. Drizzle it over the whole pan, or portion the sauce on each plate.