Place potatoes, carrots and celery in a 5-qt. slow cooker. In a large skillet, brown meat in oil on all sides. Transfer meat to slow cooker.
In the same skillet, saute onion in drippings until tender. Add garlic; cook 1 minute longer. Combine the flour, sugar, brown sugar, seasonings, orange zest and cocoa. Stir in tomato sauce; add to skillet and heat through. Pour over beef.
Cover and cook on low until beef and vegetables are tender, 6-8 hours.