Prepare the Chilies: Carefully remove the stems from the dried chilies. For a milder sauce, you can also remove the seeds. Place the chilies in a heat-safe bowl and cover with 1 cup of very hot water. Let them rehydrate for about 20 minutes, or until they are soft and pliable.

Sauté Aromatics: While the chilies are rehydrating, heat a small amount of oil in a skillet over medium heat. Add the chopped onion and garlic and cook for 3-4 minutes, until softened and fragrant.

Blend the Sauce: Once the chilies are rehydrated, transfer them to a blender along with the hot water they were soaking in. Add the sautéed onion and garlic, salt, and vinegar. Blend on high until the mixture is completely smooth. You may need to scrape down the sides of the blender a few times.

Strain and Simmer: For a smoother sauce, pour the blended mixture through a fine-mesh sieve into a small saucepan, using a spoon to press out all the liquid. Discard the solids. Bring the sauce to a low simmer over medium-low heat and cook for 5 minutes.

Adjust and Store: Taste the mexican hot sauce and adjust the seasoning if needed. You can add more salt, a little sugar to balance the heat, or more vinegar for tang. Let the sauce cool completely before transferring it to an airtight bottle or jar.