10 ounces fresh spinach (or frozen spinach that's been drained and chopped)
1 1/2 cups finely chopped onion (1 large)
1 tablespoon unsalted butter
1 teaspoon salt
1/2 teaspoon black pepper
1/4 teaspoon grated nutmeg
8 cups cubed French or Italian bread in 1-inch cubes (1/2 lb)
8 ounces coarsely grated mozzarella (3 cups)
6 ounces roasted red bell peppers, diced (from a can or jar)
2 3/4 cups milk
9 large eggs
2 tablespoons Dijon mustard