Blanch tomatoes to remove peels, then cut into cubes. Dice the onion finely with the garlic and cilantro.
Mix all ingredients (not including water) in a medium-size bowl. Use a wooden spoon or small pounder to lightly press the ingredients together in order to allow for the release of the juices.
Using a canning funnel, pour all the ingredients into the quart jar, pressing them slightly to pack and fill the jar to 1 inch below the rim. Pour in the water until it is flush with the relish ingredients within the jar.
Cover with an airlock lid or a regular ring and band lid, screwing them down tightly. Allow to sit at room temperature for 4 to 5 days or until the desired level of sourness is reached. If not using an airlock, burp the jar as needed during this time.
Once the relish is ready, keep covered and in cold storage when not in use.