Heat oven to 350°F. Place 1 pie crust in 9-inch glass pie plate as directed on box for One-Crust Filled Pie. In 12-inch skillet, melt butter over medium heat. Cook onion in butter 5 minutes, stirring occasionally, until tender. Stir in flour, salt and pepper. Cook and stir 1 minute. Stir in spinach; cook 1 minute longer.