Heat oil in a Dutch oven over medium-high heat. Add onion, red pepper and carrots. Cook, stirring often, until the vegetables are tender, about 5 minutes. Add garlic, chili powder, cumin, paprika, chipotle flakes and salt and cook, stirring constantly, for 30 seconds. Add tomatoes, stock and sweet potatoes bring to a simmer. Cover and cook until the sweet potato is tender, 15-20 minutes.