1 1/2 pounds tomatoes, diced 1/2 red onion, diced 1/2 to 1 jalapeño pepper, minced (depending on heat preference) 1/2 serrano pepper, minced 5 cloves garlic, minced 1/2 cup chopped fresh cilantro 1/2 teaspoon ground cumin 1 teaspoon pickling salt or sea salt
In a medium bowl, mix the first nine ingredients together (including the seeds and juices from your tomatoes).
Pour the salsa into a quart-sized jar and run a chopstick around the jar to release any trapped air bubbles.
Add a 1/2-inch layer of olive oil on top. The olive oil serves two purposes here: one, it prevents the vegetables from rising above the water and growing mold on the surface, and two, it adds richness to the salsa once you mix it in.
Loosely seal the jar with a lid and leave it out of direct sunlight for a few days. (I spin the lid only a couple times—enough to keep out dust and pantry pests, but loose enough to let fermentation gases escape.)