Combine first 5 ingredients, tossing to coat. Arrange mixture on a jelly-roll pan. Broil 10 minutes or until vegetables are blackened in spots. Place mixture in a blender; add cilantro, chopped oregano, cornstarch, salt, pepper, and stock. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth.