Combine flour and salt and pepper to taste. Add beef strips and toss to coat evenly. Melt 2 tablespoons butter in large skillet over medium heat. Add onion and cook until just beginning to become tender. Add mushrooms and cook briefly. Remove from pan. Add additional 2-3 tablespoons butter as necessary to saute meat. Add floured beef and cook until lightly browned. Combine beef and onion-mushroom mixture in casserole. Stir in tomato sauce and tuck in bay leaf. Cover and bake at 350'F. about 1 hour, or until meat is tender. Just before serving, stir in sour cream.