Chop the tomato and bell pepper into ½-inch cubes. Finely slice the spinach.
Into a measuring cup, pour the egg whites, chopped tomatoes, chopped bell pepper, and sliced spinach, then mix together. Sprinkle in the salt and black pepper and mix together again.
Grab a muffin tray and spray it with nonstick spray, then spoon in the egg white mixture. Place the muffin tray in the oven and cook for 20 minutes.
After 20 minutes, remove the tin from the oven and serve the egg cups immediately.