1. Heat oven to 450°F Position oven rack in the lower third of the oven.
2. Put sweet potatoes on a rimmed baking sheet. Drizzle with olive oil. Sprinkle potatoes with cumin, salt, pepper, and red pepper flakes. Toss potatoes until well coated.
3. Roast in oven 25-35 minutes, tossing potatoes once at the halfway point.
4. In a large bowl, whisk together the lime juice and olive oil. Season with salt and pepper if desired.
5. Add black beans, red onion, and cilantro. Toss gently. Add sweet potato cubes and toss gently.