4 tablespoons (1/2 stick) unsalted butter
4 cups chopped onions (2 large)
¼ cup flour
¼ teaspoon grated nutmeg
1 cup heavy cream
2 cups milk
3 pounds frozen chopped spinach, defrosted (5 (10 ounce) packages)
1 cup parmesan cheese
1 tbsp Kosher salt
½ teaspoon pepper
½ cup grated Gruyere cheese
Pre-heat oven to 425 degrees F.
Melt the butter in a heavy bottomed sauté pan (I use my big electric skillet, works great) over medium heat – add onions – sauté until translucent – around 15 mins. Add flour and nutmeg – cook and stir for about 2 mins. Add the cream and the milk – cook until thickened – stir with a whisk to ensure no clumping. Squeeze out the spinach to get all excess water out – add the spinach to the sauce. Add ½ cup of the parmesan cheese and mix well. Season to taste with salt and pepper.
Transfer the spinach to a baking dish and sprinkle with the Gruyere cheese and then the remaining parmesan. Bake for 20-25 minutes until hot and bubbly.