Roast whole sweet potatoes on a baking sheet lined with parchment paper in the preheated oven (400°F/ 200°C) until fork tender. The roasting time depends on their size; it can be anything from 30 minutes to one hour. If they are small, check after about 30 minutes and keep checking until they are tender. If you have a steamer, you can also steam the cubes; it will take about 30 minutes but check.