In a small skillet over medium heat, cook egg whites (almost like you would cook a fried egg - I cooked them two egg whites at a time).
Top the bottom half of each bagel thin with an equal amount of spinach, egg whites, tomato slices, avocado slices, mozzarella cheese, tomatillo salsa. Top with the other half of the bagel thin.
Notes
We like to use bagel thins for these sandwiches (this isn't sponsored, we just really like them!). They're fewer calories and their "Everything" flavor is one of our favorites.If you're not a fan, you could also use a regular bagel (although your calories will increase), toasted bread (have you tried Dave's Killer Bread? It's one of our favorites!), or throw the ingredients in a bowl like a scramble and eat it with a fork for a low-carb option.