If you have a molcajete, a traditional Mexican mortar and pestle, you can grind the ingredients by hand. Serve with our Pescado Tikin Xik.
Heat a small skillet over medium heat. Add tomatoes; roast until softened and charred, about 12 minutes. Transfer to a plate; set aside. In same skillet, roast serrano until charred, about 8 minutes. Transfer to a bowl; cover with plastic wrap, and let stand 10 minutes.
Peel tomatoes and pepper. Roughly chop tomatoes; place in a small bowl. Mound pepper, garlic, and salt on a cutting board; chop to form a rough paste. Stir into the tomatoes; season with salt.