Heat a skillet over medium heat and spray with cooking spray.
Whisk egg whites with salt and pepper in a bowl. Pour egg mixture into the heated skillet and swirl until eggs cover entire bottom of skillet; cook for 1 to 2 minutes. Arrange spinach, tomatoes, and feta cheese in the middle of the eggs. Cook until edges of eggs begin to curl up, 2 to 3 more minutes. Loosen omelet from the skillet using a spatula and fold in half; continue cooking until cheese is melted, 2 to 3 more minutes.