In a medium non stick skillet, heat oil over medium heat. Add diced pepper, onion, or mushrooms and cook until tender. Stir in leftover corn, broccoli or carrots. Pour in eggs, season with salt and pepper and stir mixture until it begins to thicken. Lower heat and cover for 3-5 minutes until mixture is firm. Uncover and sprinkle with cheese. Slide skillet under broiler to brown top of frittata. Note: If the skillet's handle is plastic, cover it with a double layer of foil.