3 tbsp. unsalted butter
1½ cups onion, finely chopped
2 (10 oz.) packages frozen chopped spinach, thawed and drained
1 tsp. salt, divided
½ tsp. pepper, divided
Dash freshly grated nutmeg
8 cups cubed French or Italian bread, cut into 1-inch cubes
6 oz. coarsely grated Gruyere (about 2 cups)
2 oz. finely grated Parmesan (about 2/3 cup)
9 large eggs
2¾ cup milk
Melt the butter in a medium skillet over medium heat. Add the onions to the pan and sauté until soft, about 5 minutes. Add ½ teaspoon of the salt, ¼ teaspoon of the pepper, and the nutmeg, and continue to cook for 1 minute more. Stir in the spinach, remove from the heat and set aside.
Butter the inside of a 2½-3 quart baking dish. Layer the bottom of the dish with one third of the bread cubes. Top with one third of the spinach mixture and one third of each of the cheeses. Repeat these layers twice more with the bread, spinach and cheese.
In a medium bowl, combine the eggs, milk, the remaining ½ teaspoon of the salt and ¼ teaspoon of the pepper. Whisk together until blended. Pour the mixture evenly over the bread and spinach layered in the baking dish. Cover with plastic wrap and chill for at least 8 hours or up to 1 day.
Remove from the refrigerator 30 minutes before baking. Preheat the oven to 350˚ F. Bake uncovered until puffed, golden brown and cooked through, 45-55 minutes. Let stand at least 5 minutes before serving.