Preheat the oven to 180°C fan/gas mark 6 and greaseproof a 6 hole muffin tin and set aside.
In a jug add the eggs and milk, season with the salt and pepper and whisk together.
Over a low heat, melt the butter in a small frying pan and fry the onions until they begin to soften. Remove from the heat and set aside.
Divide the onions, diced tomatoes and cheese between the 6 holes in the muffin tin. Pour the egg mixture evenly over the ingredients and give each one a little stir. Place the remaining halves of tomatoes (cut face upwards) on top of the mixture, place in the oven to cook for around 15-20mins until risen and golden. Don't worry too much about how high they rise, they will deflate when removed from the oven.
Leave to cool slightly, then lift out and enjoy warm — or chill/freeze for later. They can be eaten cold, but if you wanted to reheat them, place in the microwave, for around 30 seconds (ensure they are fully warmed through before eating).
