10 oz Frozen Spinach 1 1/2 c Onion, finely diced 3 Tbsp Unsalted Butter 1 lb  Ham, 1 inch cubes 1 tsp Salt 1/2 tsp Pepper 8 c French Bread, 1 inch cubes 2 c 6 oz Gruyère, grated 1 c 2 oz Parmigiano-Reggiano, finely grated 2 3/4 c Milk 9 Eggs, large ½ tsp Dry Mustard

Drain spinach. Drain the thawed spinach, using your hands to squeeze as much water from it as possible. Once the spinach is drained finely chop it.