Preheat the oven to 450F. Lightly oil a baking sheet.

Peel the sweet potatoes and cup into 1-inch cubes. Stem and seed the peppers and cut into 1-inch pieces. Peel the onion and cut it stem end to root end inot thin wedges. In a bowl, toss the begetables with the garlic, oil, cumin, salt, and pepper. Spread on the prepared baking sheet and roast in the oven for about 10 minutes. Stir and continue to roast for another 10 to 15 minutes, until the sweet potatoes are tender but not mushy.

While the vegetables roast, puree the tomatoes, chipotles, and cilantro in a blender until smooth. Set aside. When the vegetables are tender, put them into a 2- to 3-quart baking dish, stir in the tomato-cilantro sauce and the beans, and return to the oven until hot, about 10 minutes.

A few minutes before serving, warm the bread in the oven. Serve the stew in bowls topped with yogurt or sour cream, with warm flatbread on the side.