Scrub sweet potatoes, place on middle rack of oven and bake at 400°F for 45 minutes or until completely soft.
Meanwhile, in a large saucepan, sauté diced onions and peppers in oil over medium-low heat until onions begin to look translucent. (If you're in a hurry, you can skip the sauteing and just load the sweet potato up with raw onions and garlic, but I prefer them sauteed.)
Add garlic and sauté for one additional minute.
Add beans and seasonings and cook until thoroughly heated.
Assemble sweet potatoes: Slice baked sweet potatoes lengthwise down the middle. Fill each sweet potato with 1/4 of the bean mixture. Top with tomatoes, green onions, and sour cream.