Preheat the oven to 400°F (200°C).
Wash and peel the sweet potatoes, then cut them into 1-inch cubes.
Spread the sweet potato cubes on a baking sheet, drizzle with olive oil, and season with salt and pepper.
Roast the sweet potatoes in the preheated oven for 25–30 minutes, or until they are tender and slightly charred.
In a large mixing bowl, combine the roasted sweet potatoes, black beans, brown rice (or quinoa), cumin, chili powder, salt, and pepper.
Lay a tortilla flat and add a generous scoop of the filling in the center.
Fold in the sides of the tortilla and roll it up tightly from the bottom.
Optionally, place the rolled burritos seam-side down in a lightly greased pan and crisp them over medium heat until golden brown, about 2–3 minutes per side.
Serve warm, garnished with avocado slices and fresh cilantro if desired.