1Heat oven to 350°F. Make 1 pie crust in 9-inch glass pie plate as directed on box for One-Crust-Filled Pie. Using fork, poke holes in the base all over and bake according to package directions.
2Meanwhile in medium skillet, heat oil. Add mushrooms and red onion slices; cook over medium-high heat until soft, about 4 to 5 minutes. Cool slightly.
3In medium bowl, combine milk, spinach, minced onion, egg whites, eggs, salt and pepper; stir until all ingredients are well combined.
4Stir in Gouda cheese, Cheddar cheese and cooled mushrooms mixture. Pour egg mixture into pie shell. Sprinkle Gorgonzola cheese over top. Bake 35 to 40 minutes or until center is puffed and light golden brown. Cool 10 minutes.