In a small skillet, heat the oil over medium heat. Add the onions, tomatoes, spinach and a pinch salt. Cook until the onion is soft, about 3 to 5 minutes. Add pepper, to taste, and another pinch salt; and cook for another minute. Remove the spinach mixture from the heat to a bowl. Cover and keep warm.
In a medium bowl, whisk the egg whites, water, and a pinch salt and pepper until frothy. Lightly coat a medium nonstick skillet or omelet pan with cooking spray and heat the skillet over medium heat. Add 1/4 of the egg whites, swirling to evenly cover the bottom of the pan. Cook until set, about 1 1/2 to 2 minutes. Using a rubber scraper lift the eggs up and let the runny uncooked egg flow underneath. Spoon 1/4 of the spinach mixture onto half of the omelet, fold over, and slide onto a serving plate. Repeat with remaining egg whites and spinach mixture.
This recipe was provided by professional chefs and has been scaled down from a bulk recipe provided by a restaurant. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.