Preheat the oven to 350°F. Grease a muffin pan with cooking spray and set aside.
In a large bowl, whisk together the eggs, milk and black pepper. Then, add in the onions, spinach and tomatoes and stir well.
Pour the mixture evenly into the muffin pan.
Bake for around 20-25 minutes in the oven, or until the eggs are cooked and the tops of the muffins are firm.
Remove the muffins from the oven and allow them to cool for a few minutes.
Optional: sprinkle on some salt and add cheese or any other topping of your choice.
Serve and enjoy!