1. Chop the onions, garlic, and chili finely.
2. In a pot, combine diced tomatoes, onion, garlic, chili, vinegar, salt, and pepper. Stir over medium heat until boiling.
3. Cook over low heat for about 10 minutes until the mixture becomes smooth and thick.
4. Blend briefly, leaving it slightly chunky, and pour into sterilized glass jars. Seal tightly and let cool.