Gather all ingredients on a clean cutting board.
Dice the tomatoes into small, even pieces.
Finely chop the red onion and add it to a mixing bowl.
Seed and mince the jalapeño according to your spice preference.
Chop the fresh cilantro and add it to the bowl.
Add the diced tomatoes to the bowl with onions and jalapeño.
Squeeze lime juice over the mixture.
Sprinkle in salt and ground cumin if desired.
Mix all ingredients thoroughly.
Taste and adjust seasoning as necessary, then serve.