1. Preheat oven to 500°. Cut tomatillos in half or quarters, depending on size.
2. Spread chiles in a single layer on a baking sheet; toast 1 to 2 minutes or until chiles puff and become fragrant. (Chiles burn quickly, so keep a watchful eye.) Cut chiles open with kitchen shears; remove seeds. Cut off stems.
3. Sauté garlic in hot oil in a large saucepan over medium-high heat 2 minutes or until garlic begins to brown. Add tomatillos, salt, and 1 qt. water. Bring to a boil, and cook, stirring often, 4 minutes. Add chiles; cook, stirring occasionally, 5 to 10 minutes or until tomatillos are soft and peppers have rehydrated. Cool 10 minutes.
4. Purée mixture, in batches, in a blender or food processor. Pour through a fine wire-mesh strainer into a container, discarding solids. Cool completely. Refrigerate in an airtight container up to 4 days.