Grease a 9 cm (3.5 inches) ramekin with olive oil, so that the microwave egg muffin will not stick to the sides.
In a bowl, beat the egg with the egg whites, the salt and the pepper to your liking. Once beaten, mix in the diced tomato, the diced onion and any additional ingredient you may like (such as bacon or cheese).
Transfer the egg mixture to the prepared ramekin and microwave on high for 50 seconds to 1 minute 30 seconds, or until it will be firm to the touch on top. Make sure to keep an eye on the egg while it is in the microwave, as it may spill out of the ramekin. In this case, stop, wait and restart the microwave until cooked.
Let the microwave egg muffin cool before taking it out of the ramekin and serving.