Arrange chiles on a large baking sheet and cook in a 300 oven until chiles smell toasted (about 4 minutes). Let cool slightly. Discard stems and seeds. In a 3 to 4 quart pan, combine chiles, water, onion and garlic; cover and bring to a boil over high heat. Reduce heat, cover and simmer until chiles are very soft (about 30 minutes). Remove from heat and let cool slightly. In a blender or food processor, whirl chile mixture until smooth. Rub puree through a fine strainer and discard residue. Makes about 2 cups.