Preheat the oven to 400° F. Toss the cubed sweet potatoes with 1 tablespoon of olive oil and some salt and pepper on a sheet pan. Arrange the potatoes in a single layer; roast until just tender, about 25 minutes. Let cool slightly and then transfer to a large bowl. Add the bell pepper, scallions and cilantro.

In a small bowl, whisk the remaining 3 tablespoons olive oil, the lime juice and zest, chili powder, cumin, a generous pinch of salt, and a couple of grinds of black pepper. Pour the dressing over the salad; toss very gently. Serve warm, at room temperature, or chilled. This dish can be assembled a day in advance. Some think it improves with a day of melding in the fridge.