Place the onions and 1 teaspoon of the oil in a 10- to 12-inch nonstick skillet. Cover and cook on medium heat, stirring occasionally, for about 5 minutes, until the onions are soft and golden. Add the bell peppers (and chile pepper, if desired) and saute on medium heat for 5 minutes. Stir in the corn, black beans, and cilantro. Cover, lower the heat, and cook for about 5 minutes, until hot. Remove the skillet from the heat.
In a large mixing bowl, beat the eggs, egg whites, and salt until fluffy. Stir the bean and corn mixture into the eggs.
Coat the bottom of the same skillet with the remaining teaspoon of oil and return it to medium heat. When the skillet is hot, pour in the egg mixture and distribute the corn and beans evenly in pan. Cover and cook for 5 to 7 minutes, until the edges are firm and the bottom has browned.
Place a large, flat plate or pizza pan over the skillet and flip the skillet over so that the frittata falls onto the plate. Slide the frittata back into the skillet and cook for about 5 minutes more, until the eggs are fully cooked.
Cut the frittata into wedges and serve hot or at room temperature, topped with salsa and a dollop of sour cream or yogurt, if desired.