Place beans in a large bowl and cover by at least 2 inches of water. Let soak from 8 to 24 hours in the refrigerator. Reserve the seasoning packet.
Drain and rinse the beans, put in a soup pot with 6 cups water and the seasoning packet. Bring to a boil, cover the pot, reduce the heat and let the beans simmer for 2 hours or until tender and almost all of the liquid has cooked off.
30 – 40 minutes before the beans are done, place the chopped sweet potatoes in a large pot of cold, salted water. Bring to a boil and cook until tender, about 25 minutes. Drain the water from the potatoes, return them to the hot pot and mash with a potato masher. Add the butter and salt and pepper to taste. If the potatoes are not creamy enough, add a tablespoon or two of milk or dairy free milk and mash to desired consistency.
While the potatoes are boiling, heat a large skillet over medium-high heat, add the onions and cook until they start to soften, about 3 minutes. Add the bell and poblano peppers and the sliced mushrooms. Cook, stirring often, until the vegetables have softened and are starting to brown, about 10 minutes. Add the corn, cook for another minute, then add the beans and any of the bean liquid to the vegetables. Add the taco sauce. Raise the heat and bring to a boil. Whisk the cornstarch (or tapioca starch or arrowroot powder) with 3 tablespoons of water, stir into the vegetable mixture and boil for 1 minute. Taste and adjust seasoning with more salt and pepper if needed.
To serve, spoon some of the mashed sweet potatoes into a large, shallow bowl and top with some of the bean stew.