Preheat oven to 350°. Whisk together eggs and salt., In an 8-in. ovenproof skillet coated with cooking spray, heat oil over medium heat; saute onion and zucchini until onion is crisp-tender. Pour in egg mixture; cook until almost set, 5-6 minutes. Sprinkle with cheese., Bake, uncovered, until cheese is melted, 4-5 minutes. If desired, sprinkle with pepper.