Heat half of the oil in a cast-iron skillet over medium heat. Add chiles and garlic and cook, stirring, about 5 minutes (chiles will be puffed and garlic browned). Set garlic aside.
Transfer chiles to a bowl and cover with hot water. Let soak until soft (about 15 minutes). Drain and remove stems, seeds and veins.
Purée chiles, garlic and 2 cups water (I use the chile-soaking water).
Heat the other 3 tablespoons of oil in the same skillet over medium-high heat. When the oil is hot, add the pureed salsa and cook, stirring occasionally, until thickened, about 10 minutes. Season to taste with salt.