Chop jalapeno and serrano peppers, retaining seeds and membranes. Place into a blender with garlic, brown sugar, salt, and water. Blend until smooth, pulsing regularly.

Transfer puree into a large jar or jug. Cover it with plastic wrap or lid and place in a cool dark cupboard for 3 to 5 days, stirring once a day. The mixture will begin to bubble and ferment. Scrape down the sides during each stirring. Rewrap or seal after every stirring and return to a cool, dark place until mixture is bubbly.

Pour fermented mixture back into blender with vinegar, and blend until smooth. Strain mixture through a fine mesh strainer into a saucepan, pushing as much of the pulp as possible through the strainer into the sauce. Discard remaining pulp, seeds, and skin left in strainer.

Place saucepan over a medium-high heat and bring sauce to a boil, stirring often for 5-10 minutes, until reduced to your desired thickness. If foam develops on the top then feel free to skim this.

Remove saucepan from heat and let sauce cool to room temperature, which help the sauce will thicken a little more. Transfer sauce to your choice of jar or bottle and refrigerate.