1 (10 ounce) package frozen spinach, thawed, squeeze of all excess liquid, and chopped
1½ cups finely chopped onion (1 large)
3 tablespoons unsalted butter
1 teaspoon salt
½ teaspoon black pepper
¼ 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)
2¾ cups milk
9 large eggs
2 tablespoons Dijon mustard
Instructions
Sauté onion in butter in a large, heavy skillet over medium heat until soft, about 5 minutes. Add ½ teaspoon salt, ¼ teaspoon pepper and nutmeg and continue cooking for one minute. Stir in spinach, remove from heat and set aside.
Spread one third of the bread cubes in a well-buttered 3-quart gratin dish or other ceramic baking dish. Top with one-third of bread cubes,one-third of spinach mixture and one-third of each cheese. Repeat layering twice with remaining bread, spinach and cheese.
Whisk eggs, milk, mustard and remaining ½ teaspoon salt and ¼ teaspoon pepper together in a large bowl and pour evenly over strata. Cover with plastic wrap and chill strata for several hours or up to a day.
Remove from the refrigerator and let it stand at room temperature for 30 minutes while preheating the oven to 350°F. Bake strata, uncovered, in middle of oven until puffed, golden brown, and cooked through, 45 to 55 minutes. Let stand 5 minutes before serving.
Recipe by In Good Taste at https://ingoodtaste.kitchen/spinach-and-cheese-strata/