Chop jalapeño and serrano peppers, retaining seeds and membranes, and place into a blender with garlic, brown sugar, salt, and water. Blend until smooth, pulsing several times to start.
Transfer puree into a large glass container such as a large jar or pitcher. Cover container with plastic wrap and place into a cool dark location for 3 to 5 days, stirring once a day. The mixture will begin to bubble and ferment. Scrape down the sides during each stirring. Re-wrap after every stirring and return to a cool, dark place until mixture is bubbly.
Pour fermented mixture back into blender with vinegar; 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 on a burner and bring sauce to a boil, stirring often, until reduced to your desired thickness -about 5 to 10 minutes. Skim foam if desired.
Remove saucepan from heat and let sauce cool to room temperature. Sauce will thicken when cooled. Transfer to jars or bottles and refrigerate.