Toss stew beef with flour and 1/4 teaspoon of salt and pepper. Heat vegetable oil in large skillet, brown beef and onion. (Cooking beef and onion is optional if you have the time. It does add more flavor)) Place sweet potato cubes in the crockpot; add beef and onions. Mix beef broth with cinnamon and pour over beef mixture. Cover and cook on low for 8 to 10 hours. Add green beans during the last 15 to 20 minutes.