Preheat the grill to medium.
In a small ovenproof frying pan, heat the butter until bubbling. Pour in the egg and gently pull the mixture from the outside to the centre with a spatula, tipping the pan to allow the raw egg mixture to flow to the edges again. When it begins to set, tip in the tomatoes and half the grated Cheddar.
Flip one edge of the omelette over the other half, sprinkle over the remaining cheese and place under the grill. Heat until the cheese is golden and bubbling.
To serve, cut in half and garnish with a few cucumber slices.