Toast the chiles in a dry frying pan. Rehydrate them in 1 quart of warm water for 20 min. Drain chiles, reserving 1 cup of the water. Transfer to blender. If the chiles do not taste bitter, add the reserved chile water, otherwise add 1 cup plain water.
Sauté the onion for 5 minutes over medium heat and add to the chiles. Add the tomato, garlic, and salt, then purée.
Servings: