Place the cooked rice or quinoa in a serving bowl. Top with the shredded chicken breast and set aside.
Heat 1 tablespoon olive oil in a nonstick skillet over medium heat. Add the chopped spinach and cook for about 1 minute until slightly wilted.
Pour the egg whites into the skillet with the spinach. Add the garlic powder, salt, and black pepper. Stir gently and cook for 2–3 minutes, or until the egg whites are fully cooked and no liquid remains.
Add the diced tomatoes to the skillet and cook for 30 seconds, just until slightly warmed.
Spoon the egg white mixture over the rice or quinoa and chicken in the bowl.
Serve immediately while warm and enjoy your high-protein chicken egg white bowl.