Place chiles and 2 cups water in a medium saucepan; bring to a boil. Reduce heat; simmer, uncovered, 10 minutes. Add tomatillos; cover and simmer 5 minutes. Remove chiles and tomatillos from water; cool. Reserve water.
Process cooled chiles and tomatillos in a blender 30 seconds or until smooth, stopping twice to scrape down sides. Pour sauce into a bowl; add remaining ingredients. Stir with a wire whisk.
Note: For a thinner sauce, add some of the reserved cooking water. Recipe courtesy of Chef Christine Keff, Fandango, Seattle, Washington.
