Boston Market's Creamed Spinach

3 tablespoons butter
4 tablespoons flour
1/4 teaspoon salt
1 cup whole milk
1/2 cup sour cream
2 tablespoons butter
2 tablespoons minced onion
1/4 cup water
20 ounces frozen spinach, chopped

Over medium low heat, melt butter in a saucepan. Stir in flour and salt until creamed together. Stir in milk a little at a time. Increase to medium heat. Constantly stir with a whisk until mixture becomes thick and smooth. Remove from heat and set aside.

Place butter in a 2-quart saucepan over medium heat, add onions, and cook until the onions are transparent. Add spinach and water to pan, lower the heat and cover. Stir several times until the spinach is almost completely cooked.

When spinach is almost done, add white sauce and sour cream. Stir well and simmer until completely blended.