Heat olive oil in a 10-inch skillet over medium heat. Cook and stir broccoli, bell pepper, and sweet onion in hot oil until hot, about 3 minutes. Place a cover on the skillet and continue cooking until the vegetables begin to soften, about 5 minutes more. Stir olives into the vegetable mixture.
Beat eggs, egg whites, milk, salt, and pepper together with a whisk in a small bowl; pour over the vegetable mixture in the skillet. Sprinkle feta cheese over the egg mixture.
Replace cover on the skillet, reduce heat to medium-low, and cook until the egg is lightly browned on the bottom, 3 to 5 minutes. Carefully flip the frittata and cook until the bottom is again lightly browned, 1 to 2 minutes more.
