Wash and roughly chop the serrano peppers, removing the stems. You can keep seeds for more heat, or remove them for a slightly milder sauce.

In a clean, sterilized glass jar or container (at least 1.5-quart capacity), dissolve the salt in the unchlorinated water to create a brine, stirring well.

Add the chopped peppers to the brine, ensuring they are fully submerged. Use a fermentation weight, a small glass ramekin, or a clean, small bag filled with some brine to keep the peppers below the liquid surface. This prevents mold growth.

Cover the jar loosely with a lid, an airlock, or a coffee filter secured with a rubber band. This allows gas to escape while keeping contaminants out.

Place the jar in a cool, dark place (ideally 60-75°F or 15-24°C) for 5-7 days, or longer for more tang and complexity. Check daily for activity (bubbles) and burp the jar if not using an airlock.

Once fermented to your liking, carefully strain the peppers, reserving about 1/2 to 1 cup of the brine.

Transfer the fermented peppers to a blender or food processor. Add a small amount of the reserved brine (start with 1/4 cup) and blend until smooth. Add more brine as needed to achieve your desired consistency.

Taste and adjust seasoning if necessary. For a super smooth sauce, you can pass it through a fine-mesh sieve, pressing out all the liquid. Pour into clean, sterilized bottles or jars and store in the refrigerator for up to several months.