Arrange chiles in a single layer on a baking sheet coated with cooking spray; broil 1 minute.
Reduce oven temperature to 300°.
Place chiles, onion, and the next 5 ingredients (through chicken broth) in a blender; process until smooth. Combine pork and chile mixture in a 13 x 9-inch baking dish coated with cooking spray. Cover and bake at 300° for 2 1/2 hours. Uncover and bake an additional 30 minutes or until sauce is thick.