1 1/2 lbs red jalapeños (or serranos), stems snipped off, leaving green tops intact
6 cloves garlic, peeled
4 tablespoons light brown sugar
1 tablespoon Kosher salt
1 tablespoon fish Sauce
1/2 cup distilled white vinegar
Place peppers, garlic, sugar, and salt in bowl of a food processor fitting with steel blade. Pulse until chilies are very finely chopped, stopping to scrap sides of bowl as necessary. Transfer mixture to a clean jar, cover, and let sit at room temperature.
Check jar each day for fermentation, when little bubbles start forming at bottom of jar, about 3-5 days. Stir contents each day, continuing to let ferment until chilies are no longer rising in volume, an additional 2-3 days.
Transfer chilies to jar of a blender or food processor, add in white vinegar, Fish sauce and puree until completely smooth, 1-3 minutes. Transfer to a mesh strainer set atop of a medium saucepan. Strain mixture into saucepan, using a rubber spatula to push trough as much pulp as possible, only seeded and larger pieces of chilies should remain in strainer.Use a food mill with the medium disc to make this easier.
Bring mixture to a boil, reduce heat, and simmer until sauce thickens and clings to a spoon, 5 or 10 minutes. Transfer to an airtight container and store in refrigerator for up to 6 months.