Preheat oven to 350°. In a large skillet, heat oil over medium heat. Add onion and red and green peppers; cook and stir 4-5 minutes or until crisp-tender. Add mushrooms; cook 2-3 minutes or until tender. Remove from heat., In a large bowl, whisk eggs, milk, flour, baking powder and seasonings until blended. Stir in cheese and vegetable mixture. Transfer to a greased 13x9-in. baking dish. Bake, uncovered, 30-35 minutes or until set.