Combine the jalapeños, garlic, onions, salt, and oil in a nonreactive saucepan over high heat. Sauté for 3 minutes. Add the water and continue to simmer, stirring often, for about 20 minutes. Remove from the heat and allow to steep until mixture comes to room temperature.  

In a food processor, puree the mixture for 15 seconds, or until smooth. With the processor running, pour the vinegar through the feed tube in a steady stream. Pour into a sterilized pint jar or bottle and secure with an airtight lid. Refrigerate.  

Let age for at least 2 weeks before using. Can be stored in the refrigerator up to 6 months.