Arrange 2 bread slices in a single layer in a 2-quart soufflé dish coated with cooking spray. Sprinkle with 1/2 cup cheese, one-third of onion, and one-third of chiles. Repeat layers twice.
Combine milk and next 6 ingredients (milk through eggs) in a blender, and process until smooth. Pour over bread layers. Cover with plastic wrap, and refrigerate 8 hours.
Preheat oven to 350°.
Uncover; sprinkle strata with pumpkin seeds, if desired. Bake at 350° for 1 hour and 5 minutes or until a knife inserted in center comes out clean. Let stand 10 minutes before serving.