Heat the oil in a frying pan and fry the tomatoes (you may have to do this in 2 batches) for 2–3 minutes until they have softened. Add the basil and season with salt and pepper, then transfer to a bowl and keep warm.
Beat the eggs with the mustard and season with salt and pepper.
Melt a quarter of the butter in an omelette pan or small frying pan until it stops foaming, then swirl in a quarter of the egg mixture. Fork over the omelette so that it cooks evenly. As soon as it is set on the bottom (but still a little runny in the middle), dot over a quarter of the goats' cheese and cook for a further 30 seconds. Carefully slide the omelette on to a warmed plate, folding it in half as you do so.
Repeat with the remaining mixture to make 3 more omelettes. Serve with the tomatoes and garnish with watercress.