Remove the stems from the Fresno chilies and red bird’s eye chilies. Place the chilies in a food processor or mortar and pestle along with the vinegar, garlic cloves (if using), and water. Blend until a chunky paste forms, with visible seeds throughout.
Transfer the puréed mixture to a saucepan and bring to a boil. Reduce the heat and simmer for 8-10 minutes until the pepper pieces have softened and the sauce is thicker.
Stir in the salt.
Remove from the heat and let the sauce cool. Transfer the sauce to sterilized jars. Store in the refrigerator, where it will keep for up to three weeks.
