Pre-heat oven to 190C Fan and lightly grease an ovenproof dish.
Beat the eggs and milk together.
Stir in the grated cheese and diced onion.
Layer the tomatoes over the bottom of the greased ovenproof dish and pour over the egg mixture.
Bake in the centre of the oven for 30 to 40 minutes, test the omelette is fully cooked by inserting a skewer / cocktail stick into the centre which should come out clean if the omelette is cooked through.