Preheat the oven to 450 degrees F (230 degrees C).

Drizzle sweet potatoes with 1 tablespoon olive oil on a rimmed baking sheet; season with cumin, pepper flakes, salt, and black pepper. Toss until evenly coated.

Roast in the preheated oven on the lower rack, stirring halfway through, until sweet potatoes tender, 25 to 35 minutes.

Whisk remaining 2 tablespoons olive oil and lime juice together in a large bowl; season with salt and black pepper. Add sweet potatoes, black beans, onion, and cilantro; gently toss to coat.