Add the spinach, tomato, red bell pepper, and onion to a large bowl.
In a separate large bowl, whisk the eggs just until the yolks and whites are blended. Add the salt, pepper, and garlic powder to the eggs. Blend until combined.
Spray a nonstick muffin tin with nonstick cooking spray. Divide the spinach, tomato, red bell pepper, and onion mixture evenly between the 12 muffin tins.
Carefully pour the egg mixture into each muffin tin. Fill each tin about 75% full with egg mixture, submerging the vegetables already in the tin.
Sprinkle some cheese over each egg muffin before placing them in the oven. Bake for 20 minutes or until a toothpick comes out clean when inserted into the center of an egg muffin. Enjoy!
To reheat egg muffins, microwave each muffin on high for 30 seconds or until it is heated through.