4 tbsp. (1/2 stick) unsalted butter
4 c. chopped yellow onions (2 large)
1/4 c. flour
1/4 tsp. grated nutmeg
1 c. heavy cream
2 c. milk
3 lb. frozen chopped spinach, defrosted (five 10 oz. pkg.)
1 c. freshly grated Parmesan cheese
1 tbsp. kosher salt
1/2 tsp. freshly ground black pepper
1/2 c. grated Gruyere cheese

Preheat the oven to 425°F. Melt the butter in a heavy-bottomed saut pan over medium heat. Add the onions and saut 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 mixture 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.