This is Eduardo Sol's grandmother's recipe from Guadalajara.

4 ripe tomatos
5 chile serranos
2 cloves garlic
1 teaspoon rock salt
1 lime
2 tablespoons water (if the tomatos are not too jucy)

Wrap the tomatoes, chile serranos, and 2 cloves of garlic in aluminum foil. Grill or cook them for 7 minutes.
Immediately empty the contents of foil into a blender or food processor with the rest of the ingredients.

Note on blender use: You will want to use the "pulse" function to make a traditional dip which is chunky, giving it a nice traditional look. If you are going to use this as a sauce blend it thoroughly to homogenize the taste and give it a normal sauce look.