1 pound bag chopped frozen spinach, thawed
3 tablespoons unsalted butter
1 large yellow onion, chopped (about 2 cups)
3 garlic cloves, minced
12 ounces (3/4 pound) Italian sandwich bread, cut into 1-inch cubes (about 11 slices or 8 cups)
6 ounces (1-1/2 cups) coarsely grated Gruyère
2 ounces (2/3 cup) finely grated Parmigiano-Reggiano
9 large eggs
2-3/4 cups half-and-half
2 tablespoons Dijon mustard
1-3/4 teaspoons salt
3/4 teaspoon black pepper
1/4 teaspoon ground nutmeg
Squeeze handfuls of spinach to remove as much liquid as possible.
Melt the butter in a large sauté pan over medium heat. Add the onion and cook, stirring frequently, until soft and translucent, about 5 minutes. Add the garlic and drained spinach and cook 2 minutes more. Set aside.
Butter a 3-quart (or 9x13-inch) baking dish. Spread one third of the bread cubes in the dish and top evenly with one third of spinach mixture. (Try to break it up as much as possible so there are no huge clumps of spinach.) Sprinkle with one third of each cheese. Repeat layering twice, ending with cheeses.
In a large bowl, whisk the eggs. Add the half-and-half, Dijon mustard, salt, pepper and nutmeg and whisk until well combined. Pour the custard mixture evenly over the strata. Cover the strata with plastic wrap and chill for at least 1 hour or overnight.
Preheat oven to 325°F. Bake the strata, uncovered, until puffed, set and golden brown all over top, 70-75 minutes. Let stand 10-15 minutes before serving.
Note: The cook time is based on the strata going immediately from the fridge to the oven. If it sits out at room temperature before baking, the cook time will be 10-15 minutes less.