Coat a 13-by-9-inch baking dish with cooking spray. Chop the Canadian bacon or ham and cut the English muffins into cubes.
Brown the Canadian bacon in a skillet over medium heat, then set it aside. Ham may be browned or used as is.
Scatter half the meat in the dish, add all the English muffin cubes, and top with the remaining meat.
Whisk the eggs, milk, cream, salt, and pepper together. Pour the mixture over the casserole, pressing the muffin cubes down so they are coated. Cover and refrigerate at least 4 hours or overnight.
The next day, heat the oven to 350°F. Let the casserole sit out while the oven heats, then bake covered with foil for 45 minutes.
Remove the foil and bake 15 minutes more, until the center is set and a toothpick comes out clean.
For the hollandaise, melt the butter over medium-low heat. Whisk in the cream, egg yolks, lemon juice, hot sauce, and mustard until thickened; do not let it boil.
Taste and adjust the hollandaise, drizzle it over the hot casserole, and garnish with dill or green onion if desired.