Add bell peppers to pot and cook for about 3 minutes, stirring occasionally. Stir in toma-toes, broth, chipotle pepper, and adobo sauce. Bring to a boil and add flour mixture to thicken. Add beef and bring to boil. Reduce heat and simmer until beef is hot.
Meanwhile prepare Chipotle Sweet Potatoes: Place sweet potatoes in large saucepan; cover with water. Bring to a boil; cook 12 to 15 minutes or until tender. Drain, reserving 1/8 cup of the cooking liquid. Return potatoes and reserved cooking liquid to saucepan; add butter, chipotle pepper, adobo sauce, salt, and ancho chili pepper. Mash potatoes. Serve stew over sweet potatoes.
Tip: for less heat, remove seeds from chipotle peppers.