In a large mixing bowl, add tomatoes, onion, garlic, cilantro, lemon juice, salt and jalapeño, if using. Mix everything together.
Place salsa into glass jars. Make sure everything is submerged in liquid. Loosely cover jars with lid.
Place jars on a large dish to catch any spillover. Let salsa sit out on your counter for 3-4 days to ferment. Check everyday and push down salsa with a clean fork.
After 3-4 days, once the salsa has reached your desired taste, cover the lids tightly. Place salsa in the refrigerator.