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Mix the tomatoes, peppers, onion pieces, garlic, and cumin together in a two-quart mason jar. Add the lime juice.
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In another container, dissolve the salt in the water. Pour the brine over the vegetables, and weight them. Cover the jar (use an airlock, if you have one), and keep the jar at cool room temperature. Skim off any yeast or mold that appears.
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After three weeks, cut a pepper vertically to be sure it has completely changed color, from bright green to olive green. If it has, gently transfer all the vegetables to a bowl, taking care not to burst the tomatoes, which will have swelled.
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Coarsely grind the vegetables, in batches, in a food processor or blender. Transfer the mixture to one or more smaller jars, and store them in the refrigerator. The salsa should keep for weeks.