To prepare the sweet potatoes, preheat the oven to 400°F (200°C) and line two rimmed baking sheets with parchment paper. Put the sweet potatoes in a large bowl, drizzle with the oil, and toss until evenly coated. In a small bowl, stir together the salt, paprika, chili powder, cumin, garlic powder, pepper, and cayenne, adding a bit more cayenne if you like things spicy. Scatter the spice mixture over the sweet potatoes and toss again. Spread the sweet potatoes on the baking sheets in an even layer. Bake, stirring occasionally, for about 35 minutes, until quite crispy.
Meanwhile, make the queso sauce. Combine all the ingredients in a blender (preferably a high-speed blender) and process until totally smooth. If the sauce is thicker than you’d like, add water by the tablespoon until it's the right consistency for you.
To prepare the fixings, put the tomatoes, onion, avocado, cilantro, lime juice, agave nectar, and salt in a medium bowl and toss to combine.
Put the sweet potatoes in a large serving dish. Top with the tomato mixture, then the black beans. Finally, drizzle the queso sauce evenly over the top. Serve right away, with any other desired toppings.