Spinach Gratin

Ingredients
4 tablespoons (1/2 stick) unsalted butter
4 cups chopped yellow onions (2 large)
1/4 cup flour
1/4 teaspoon grated nutmeg
1 cup heavy cream
2 cups milk
3 pounds frozen chopped spinach, defrosted (5 (10-ounce) packages)
1 cup freshly grated Parmesan cheese
1 tablespoon kosher salt
1/2 teaspoon freshly ground black pepper
1/2 cup grated Gruyere cheese
Directions
Preheat the oven to 425 degrees F.

Melt the butter in a heavy-bottomed saute pan over medium heat. Add the onions and saute until translucent, about 15 minutes. Add the flour and nutmeg and cook, stirring, for 2 more minutes. Add the cream and milk and cook until thickened. Squeeze as much liquid as possible from the spinach and add the spinach to the sauce. Add 1/2 cup of the Parmesan cheese and mix well. Season, to taste, with salt and pepper.

Transfer the spinach to a baking dish and sprinkle the remaining 1/2 cup Parmesan and the Gruyere on top. Bake for 20 minutes until hot and bubbly. Serve hot.