In a large skillet over medium heat, heat the oil. Add the bell pepper, chili powder, and salt and cook until softened, about 3 minutes. Add the beans, tomatoes, chicken broth, lamb, and carrots. Cover and bring to a boil. Reduce heat and simmer for 25 minutes.Meannwhile, in a small bowl, whisk together the flour and 1/4 cup warm water. Stir into the chili and cook until thickened, about 5 minutes more. Stir in the scallions.