Ham & Cheese Strata

1 onion, chopped
8oz ham, chopped
1 (10oz) pkg frozen spinach, thawed & drained well
4 oz cheddar
5 slices dense white bread, halved diagonally
3 eggs
4 egg whites
2 1/2 C milk
3/4 tsp dry mustard
dash pepper

Coat small non-stick skillet and 2qt baking dish with vegetable oil spray. Heat skillet; add onion and cook 4 minutes. Add ham, spinach and cheese; mix well. Spread half of ham mixture over bottom of baking dish. Top with bread, then remaining ham. Whisk together eggs, whites, milk, mustard and pepper. Pour over bread. Cover and refrigerate 3 hours or overnight. Preheat oven to 350. Bake 40-50 minutes.