Heat the oil in a large frying pan and cook the onion until soft.

Add the diced tomatoes and cook until soft.

Add the beaten egg, spreading it across the pan and cook on medium heat until its almost set.

As it cooks draw the mixture in from the edges of the pan so you get an even thickens.

Put the pan under pre-heated grill for a couple of minutes until the omelette is firm.

Slice into wedges and serve.