Spinach & Cheese Strata - Once Upon a Chef

Ingredients

• 1 lb bag chopped frozen spinach, (thawed and squeezed as dry as possible)
• 3 tablespoons unsalted butter
• 1 large yellow onion, (chopped (about 2 cups))
• 3 cloves garlic, (minced)
• 12 oz Italian sandwich bread, cut into 1-in cubes (about 11 slices or 8 cups)
• 6 oz (1½ cups) coarsely grated Gruyère
• 2 oz (⅔ cup) finely grated Parmigiano-Reggiano
• 9 large eggs
• 2¾ cups half-and-half
• 2 tablespoons Dijon mustard
• 1¾ teaspoons salt
• ¾ teaspoon freshly ground black pepper
• ¼ teaspoon ground nutmeg