Almost any fresh red chile will do, but we have had especially good luck with what are known as “red finger hots.” Red jalapeños or Fresnos will work as well. Though you may be tempted to use red Thai chiles, fight this impulse! They are incredibly difficult to seed and, if used whole, will make the sauce way too spicy.
Wearing gloves, stem, seed, and coarsely chop:
Transfer the chiles to a medium saucepan and add:
Bring the mixture to a boil, simmer for 10 minutes, and remove from the heat. When cool, transfer to a blender or food processor and puree until smooth. Add water or a touch of vinegar if the mixture is too thick; it should be the consistency of ketchup. Season to taste with:
Transfer to a bottle or jar. Store in the refrigerator. The sauce will keep for at least 1 month.