4 tablespoons unsalted butter
4 cups chopped yellow onions
1/4 cup flour
1/4 teaspoon grated nutmeg
1 cup heavy cream
2 cups milk
5 (10 ounce) packages frozen chopped spinach, thawed
1 cup grated parmesan cheese
1 tablespoon kosher salt
1/2 teaspoon ground black pepper
1/2 cup grated gruyere cheese
Preheat oven to 425.
Melt the butter in sauté pan over medium heat.
Add the onions and sauté about 15 minutes, until translucent.
Add the flour and nutmeg and cook stirring, for 2 more minutes.
Add the cream and milk and cook until thickened.
Add salt and pepper.
Squeeze as much liquid as possible from the spinach.
Add the spinach to the sauce.
Add 1/2 cup of the cheese, mix well.
Transfer to a baking dish and sprinkle with the remaining cheese and gruyere on top.
Bake for 20 minutes until hot and bubbly.