Serves 6 to 8

1 (10 ounce) package frozen spinach, thawed, squeeze of all excess liquid, and chopped
1 1/2 cups finely chopped onion (1 large)
3 tablespoons unsalted butter
1 teaspoon salt
1/2 teaspoon black pepper
1/4 teaspoon freshly grated nutmeg
8 cups cubed French or Italian bread in 1-inch cubes (1/2 lb)
6 ounces coarsely grated Gruyère (2 cups)
2 ounces finely grated parmesan (1 cup)
2 3/4 cups milk
9 large eggs
2 tablespoons Dijon mustard