You can assemble this the night before and pop it in the oven to bake the next morning. However, I'm not that organized. I put it together in the morning and bake it as soon as it is done. Works just fine either way.
Grease a 2 qt. casserole pan with butter.
Trim crusts off bread. Cube slices and spread on shallow baking sheet. Bake or broil until cubes are dry, stir half way through so cubes are flipped over. I usually bake the bread cubes at 350 for about 10 min. You don't want croutons, just slightly dried out bread cubes.
Brown crumbled sausage until thoroughly cooked. Drain off fat.
Preheat oven to 325 degrees
Place bread cubes evenly in bottom of baking dish.
Sprinkle cheddar cheese on top of bread cubes. Add sausage on top.
Mix salt, dry mustard, pepper and onion powder together (this helps to keep the mustard from getting lumpy).
Add milk and spices to beaten eggs. Mix well.
Gently pour mixture on top of meat, cheese, bread mixture.
Bake uncovered for 1 hour. Cover with foil if the top gets too brown before center is done. (Test with toothpick or wooden skewer. It should come out clean.) Frittata will puff up in oven but sinks a bit when removed.
Serve with salsa, green onions, peppers, sour cream, etc. Very good reheated for a quick snack or lunch.