Wash all vegetables thoroughly.
Dice the ripe tomatoes and place them in a mixing bowl.
Finely chop the onion and add it to the bowl.
Seed and mince the jalapeño, then add to the mixture.
Chop the fresh cilantro and stir it in.
Squeeze the juice of one lime over the mixture.
Add salt and cumin, if using, to taste.
Mix everything well until combined.
Taste and adjust seasoning as needed.
Serve immediately or let it rest for 30 minutes for flavors to meld.