Watercress teams well with eggs and is highly nutritious, providing iron and betacarotene. The cheddar cheese supplies plenty of taste without too much fat. Serve with crusty French bread for a well-balanced meal.
Using a fork, whisk the eggs with the water and freshly ground black pepper in a bowl until frothy.
Heat the butter in an 18 cm non-stick frying pan over a medium heat until foaming. Tilt the pan to coat the bottom with the butter, then pour in the beaten egg mixture. Cook for about 1 minute, stirring gently with a spatula and pulling the cooked egg in from the edges so that the unset egg runs onto the hot pan and starts cooking.
When the egg is lightly set, stop stirring and scatter the chopped tomatoes into the pan. Cook for a further 30 seconds or until the egg is just set and golden underneath. Remove from the heat.
Sprinkle the cheese and watercress over the omelette, then slide the omelette onto a warmed plate, folding it over in half as it slides from the pan. Serve immediately.