Split the English muffins and chop them into one-inch pieces. Spread the pieces on an ungreased baking sheet and broil for 1-2 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 9×13 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 the refrigerator 30 minutes before baking. Sprinkle with paprika. Cover and bake at 375° for 35 minutes. Uncover; bake 10-15 minutes longer or until the center is set.
In a double boiler or metal bowl over simmering water, constantly whisk the egg yolks, cream, lemon juice and mustard 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 this blender method to make this hollandaise sauce (use the ingredients listed here on this recipe; add the cream to the egg yolks.)
Serve immediately with casserole. You can drizzle it over the whole pan or portion the sauce on each plate.