The spinach gratin recipe by Ina Garten is a good baked spinach casserole recipe, and it is available on the Food Network website. The recipe calls for frozen spinach, butter, onions, milk, cream, flour, cheese and several seasonings.
To prepare the spinach casserole, melt 4 tablespoons of butter in a heavy-bottomed pan, and saute 4 cups of chopped yellow onions in the butter over medium heat until they become translucent. Pour in 1/4 cup of flour and 1/4 teaspoon of ground nutmeg, and continue sauteing for 2 more minutes. Pour 2 cups of milk and 1 cup of heavy cream to the pan, and cook until thickened. After defrosting 3 pounds of frozen spinach, or five 10-ounce packages, drain as much liquid as possible from the spinach before adding it to the onion mixture.
Add 1/2 cup of freshly grated Parmesan cheese, 1 tablespoon of salt and 1/2 teaspoon of ground black pepper. Stir the entire mixture, and transfer it to a casserole dish. Top the casserole with another 1/2 cup of Parmesan and 1/2 cup of grated Gruyere cheese, and bake at 425 degrees Fahrenheit for 20 minutes or until bubbling. Serve the dish hot alongside matching food items such as chicken, sweet potatoes or butternut squash. The recipe serves eight people.