Heat the oil in a pan on a medium heat.
Dice the onion and chop the tomatoes in halves and add to the pan and fry off for 1 minute.
Meanwhile crack your eggs into a separate dish with the milk, salt and pepper and then whisk/ beat until fully combined. Add to the pan with the onions and tomato.
Tear the mozzarella up and add to the pan. Cook until you are able to flip the egg, roughly 3-4 minutes. Do not stir the egg. If you don't feel comfortable flipping the egg use a spatula or finish it off under the grill.