Preheat the oven to 425F.
Spread the sweet potato out on a rimmed baking sheet, drizzle generously with oil and toss around with your hands to coat. Sprinkle with salt and pepper and roast for 25-30 minutes, until tender and turning golden.
Set a medium pot over medium-high heat, add some oil and saute the onion , peppers and garlic for a few minutes, until soft. Chop and add the cilantro stems and about half of the leaves (save the rest to put on top) along with the chili powder and cumin and cook for another few minutes.
Add the beans and tomatoes, scrape the roasted sweet potatoes into the pot, season with salt and pepper and bring to a simmer; cook for 30-45 minutes, until thickened. If you like, continue to cook it over low heat, or cool it down and refrigerate overnight to let the flavours develop a bit. Otherwise, it's good to go. Serve hot, topped with crumbled feta (or sour cream) and fresh cilantro.