Preheat the oven to 400°F (200°C).
Peel and cube the sweet potatoes into 1-inch pieces. In a mixing bowl, toss sweet potato cubes with olive oil, ground cumin, chili powder, salt, and pepper until well coated.
Spread the seasoned sweet potatoes evenly on a baking sheet and roast in the preheated oven for about 25 minutes, or until they are tender and slightly crispy, tossing halfway through.
While the sweet potatoes are roasting, prepare the avocado lime dressing. In a separate bowl, mash one avocado with a fork. Add lime juice, olive oil, minced garlic, and salt to taste. Mix until smooth and creamy.
If using rice or quinoa, cook according to package instructions while the sweet potatoes are roasting.
Once the sweet potatoes are done, remove them from the oven and let them cool slightly.
To assemble the bowls, divide the cooked rice or quinoa (if using) into four dishes. Top each bowl with roasted sweet potatoes, black beans, diced red bell pepper, and sliced avocado.
Drizzle each bowl with avocado lime dressing and garnish with fresh cilantro, if desired.