Heat oil in a Dutch oven or similar heavy-bottom pot over medium heat. Add the onion, bell pepper, poblano peppers and a pinch of salt, and saute until onions are translucent and peppers have become tender, for about 10 minutes.
Add the chili powder, cumin, jalapeño and garlic, and cook for another minute, until the spices become fragrant.
Add the stock, along with the sweet potatoes. Cover and simmer until the sweet potatoes are tender, for about 10 minutes.
Add the beans, cover and simmer, stirring occasionally, for about 30 minutes.
Assemble the tacos with a heaping spoonful of the beans, followed by avocado, sour cream, radishes and cilantro.