Preheat oven to 400°F. Whisk together eggs, salt, and cheese in a medium bowl.
Heat oil in a 10-in. nonstick ovenproof skillet over medium-high heat. Add onion and peppers, and cook, stirring often, until softened, 8 to 10 minutes. Add cherry tomatoes, and cook, stirring occasionally, until tomatoes begin to burst, 3 to 4 minutes. Add egg mixture and cook, stirring gently, until eggs just begin to set and vegetables are evenly distributed, about 30 seconds to one minute.
Transfer pan to oven and bake until center is set, about 8 minutes. Cut frittata into 4 wedges and top with avocado and cilantro, if desired.