Cook the spinach as directed on the package; drain and press with paper towels to remove all of the moisture.

Meanwhile, heat the oil in a 10-inch nonstick skillet over medium heat. Add the onion and cook for 1 minute.

Whisk the sour cream, milk, cheese, flour and salt in a small bowl. Stir into the onions in the skillet. Cook the sauce until bubbly and thick. Stir the spinach into the sauce until well mixed and serve.