*Note: it is really important to squeeze as much liquid from the spinach as possible or the gratin will be watery. The best method I've found is to put the spinach in a large dishtowel, roll up the towel, and twist the ends to wring out the liquid. Use those muscles and get rid of all the water!
*Serves 8-12 as a side dish
4 tablespoons (1/2 stick) butter
4 cups minced yellow onions, about 2 large onions
2 cloves garlic, finely minced
1/4 cup flour
1/4 teaspoon grated nutmeg
1 cup heavy cream
2 cups milk
3 pounds frozen chopped spinach, defrosted (5 (10-ounce) packages)
1 cup freshly grated Parmesan or Parmigiano Reggiano cheese
1 tablespoon kosher salt (or 1/2 tablespoon table salt)
1/2 teaspoon freshly ground black pepper
1/2 cup grated Gruyere, Swiss or Parmesan cheese
Preheat the oven to 425 degrees F.
Melt the butter in a large skillet (12-inch) over medium heat. Add the onions and saute until translucent, about 10 minutes. Add the garlic and sauté until fragrant, about 2 more minutes. Add the flour and nutmeg and cook, stirring, for 2 more minutes. Add the cream and milk and cook, stirring often and quickly, 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 the salt and pepper.
Transfer the spinach to a 9X13-inch or similar sized baking dish and sprinkle the remaining 1/2 cup Parmesan and the Gruyere (or Swiss or Parmesan, if using) on top. Bake for 20 minutes until hot and bubbly. Serve hot.