Heat vegetable oil in a medium glass or enamel pan over high heat. Add the sliced jalapeños, minced garlic, minced onion, and salt. Cook for 4 minutes, stirring often, until the onions turn clear and the jalapeño skins soften.
Pour in the water and scrape up any browned bits from the bottom of the pan. Keep the mixture at a steady simmer for 20 minutes, stirring often, until the peppers break apart easily and the liquid reduces by about one quarter.
Let the cooked mixture cool completely to room temperature. Transfer it to a food processor and blend on high for 60 to 90 seconds until smooth. While the processor is running, slowly pour in the white vinegar and keep blending until the sauce is fully combined.
Pour the sauce into sterilized glass jars or swing-top bottles using a small funnel. Leave about ¼ inch (6 mm) of space at the top, seal tightly, and refrigerate right away. Keep the sauce below 40°F (4°C) for best storage.
