spinach and cheese strata

INGREDIENTS

• 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