For the sweet potato and squash filling: Preheat the oven to 425 degrees F.
Toss the sweet potato and squash cubes with the olive oil, cumin, garlic and salt to taste in a bowl and then transfer to a baking sheet. Bake until the vegetables are lightly browned and fork-tender, approximately 30 minutes.
For the mushroom saute: Heat the oil in a large skillet over medium-high heat and add the onions and garlic. Cook until translucent, approximately 3 minutes. Add in the mushrooms and cook, stirring, until lightly browned, about 10 minutes. Add the chile powder, then juice the lime and orange halves over the mushrooms and add salt to taste. Let sit on low heat for 1 minute. Add the cilantro and remove from the heat.
For the ground bison: In a large skillet, heat the olive oil on high heat and add the ground bison. Cook, stirring continuously, until it starts to brown. Add in the cumin, smoked paprika, cinnamon, cayenne pepper, black pepper and salt to taste. Reduce the heat to medium low and cook, stirring occasionally, until the liquid reduces and the meat is browned.
For the taco bar: Place the corn tortillas, lettuce leaves, radishes, bell peppers, avocado, cheese, pico de gallo and yogurt into separate serving containers. Place the mushrooms, bison and sweet potato and squash mixture into separate bowls. Have fun assembling!