In a large skillet, brown meat in oil on all sides; drain. Transfer to a 3-qt. slow cooker. Stir in the squash, cabbage, red pepper and celery. In a blender, combine tomatoes and brown sugar. Cover and process until blended. Pour over vegetables. Combine the broth, adobo sauce, oregano, salt and pepper; add to slow cooker. Cover and cook on low for 7-8 hours or until meat and vegetables are tender. If desired, thicken pan juices.