Break the eggs into a jug and add the water. Whisk together lightly to mix inches Stir in the thyme and a little black pepper to season. Set aside. Heat the grill to red hot. Heat the oil in a small omelette pan and over a low heat cook the onion, celery or sweetcorn and chosen filling. Cook for 4 minutes and then pour in the egg mixtue.
Turn up the heat and let the mixture run around the pan to set. Cook until the cente is almost set, then sprinkle cheese on top and quickly put the pan under the grill. Heat until the cheese is melted and the filling has risen. Slide on to a hot serving plate and serve immediately with some fresh wholewheat bread and a crisp green salad.