Spray inside of 3 1/2-quart or larger slow cooker with cooking spray. Place onion evenly on bottom of slow cooker. Top with layers of carrots, celery, potatoes and meat; sprinkle with thyme. Pour undrained tomatoes and water over meat.
Cover; cook on LOW 8 to 10 hours or on HIGH 4 to 5 hours or until meat is tender.
Stir in tomato paste; cover. Cook 10 minutes more.