1. Place jalapeño and serrano peppers into a blender with water, garlic, brown sugar, and salt. Pulse several times, them blend until smooth. 

2. Transfer puree into a large glass jar or pitcher. Cover container with plastic wrap and place into a cool dark location for 3 to 5 days, stirring and scraping down the sides once a day. The mixture will begin to bubble and ferment. Rewrap after every stirring and return to a cool, dark place until the mixture is bubbly.

3. Pour fermented mixture back into the blender; add 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 the strainer. 

4. Transfer sauce into a small saucepan; bring to a boil over medium heat, stirring often, until reduced to your desired thickness, about 5 to 10 minutes.

5. Let sauce cool to room temperature. The sauce will thicken a little when cooled. Transfer sauce to jars or bottles and store in the refrigerator.