In a mixing bowl, add egg whites, ground sea salt and freshly ground black pepper; whisk until frothy and set aside.
In a small skillet over medium heat, add butter and when hot, add onions and mushrooms; season with ground sea salt. Sauté the ingredients until nicely cooked, about 1 ½ minutes. Add tomatoes and season with hot sauce; sauté for 1 minute. Remove from the heat and transfer to a bowl.
Wipe the skillet and return to the heat; set the temperature to medium-low. Spray generously with cooking spray. Pour in egg white mixture, swirl to cover bottom evenly and cook until set, about 3 to 4 minutes or until the edges set and release from the pan, lifting with a spatula. Spoon onion mixture on half of the omelet, sprinkle cheese on top and then fold it over. Cook until cheese has melted, about 1 minute. Slide omelet onto a serving plate. Makes 1 large omelet.