Spinach Gratin - cooking recipe

Ingredients

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

Preparation

Preheat the oven to 425\u00b0.
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.