Stir egg whites with cumin. Spray a medium nonstick skillet with cooking spray and place on medium heat. Add egg whites, tilting the pan to coat evenly. Cook until bottom is set but top is still moist, 2 to 3 min.
Sprinkle half the omelet with corn, beans and green onions. Spoon half the salsa over filling; sprinkle with cheese. Use a spatula to fold the un-garnished side over the filling. Cover; remove from heat and let stand 2 min. Garnish with avocado and remaining salsa.