Cut steak into 6 equal portions. Place each piece between two layers of plastic wrap and pound with a mallet or rolling pin until about 1/4" thick. Spread each piece with mustard and set aside. Place carrots in bottom of 5-6 qt slow cooker.
Cook bacon in skillet until crisp. Drain on paper towels. Meanwhile, in bacon grease left in skillet, saute onions until tender, about 5 minutes. Remove onion to a bowl with slotted spoon. Add chopped pickles. Crumble bacon and add to onion mixture. Divide equally amongst pieces of beef; roll each piece around the filling and secure with kitchen twine. Dredge in flour, tapping off any excess.
Add butter to any remaining bacon grease. When foaming subsides, add beef rolls and brown 1-2 minutes on each side. As each is finished, transfer to the slow cooker on top of the carrots. Pour beef stock over all. Cover and cook on low 6-7 hours until carrots are tender and beef is very tender. Remove beef with tongs; carefully remove strings. Remove carrots with a slotted spoon.
Combine sour cream and flour; stir well. Add to liquid remaining in slow cooker and stir well. Turn heat to high. Return beef and carrots to pot and cover. Cook 10-20 minutes until thick. Season with salt and pepper to taste. Serve hot.