Spinach Gratin

2 Tablespoons unsalted butter
4 cups chopped onions (2 large)
¼ cup all purpose flour
2 teaspoons Kosher salt
½ teaspoon freshly ground pepper
¼ teaspoon grated nutmeg
3 cups milk
5 (10-ounce) packages chopped spinach, thawed
1 cup freshly grated Asiago or Parmesan cheese, divided
¾ cup grated Gruyère cheese

Preheat the oven to 425° F.

By the handfulls, squeeze all water from spinach. Repeat with paper towels. Set aside

Melt the butter in a heavy bottomed sauté pan over medium heat. Add the onions and sauté until translucent, about 15 minutes. Add the flour, salt, pepper, and nutmeg and cook, stirring, for 2 more minutes.. Add the milk and cook until thickened. Add the spinach to the sauce, along with ½ cup of the Asiago cheese; mix well. May be prepared in advance up to this point.

Transfer the spinach mixture to a baking dish and sprinkle the remaining ½ cup of the Asiago cheese and Gruyère cheese. Bake for 20 minutes, or until hot and bubbly. Yield: 8 servings.