Drain and rinse the tomatoes in a colander.
Cut and seed the jalapeno.
Cut the onion slices into quarters.
Throw the tomatoes, onion, jalapeno, cilantro, and cumin into a food processor.
Squeeze the juice from the lime and throw the juice into the food processor.
Blend the ingredients until the onions are in little bits.
Add a little pepper and salt to taste.
Blend again for few seconds.
Taste. Add salt and blend until you are happy with the taste.