Spinach Sourdough Strata

Ingredients

• 1 bag chopped frozen spinach, thawed, excess liquid squeezed out
• 4 tablespoons butter
• 1 large yellow onion, chopped, about 2 cups
• 8 cups stale sourdough bread cubes (1 inch or smaller cubes)
• 2 cups grated Gruyère cheese
• 1 heaping cup grated sharp cheddar cheese
• 10 large eggs
• 4 cups whole milk
• 2 tablespoons Dijon mustard
• 1 teaspoon salt
• ½ teaspoon, or more, freshly ground black pepper
• ¼ teaspoon ground nutmeg