Put the sweet potatoes in a large bowl and toss with the oil. With a mortar and pestle, coarsely crush the coriander seeds, cardamom seeds, cumin seeds, and pepper-corns. Toss the spices with the sweet potatoes to coat evenly. Cover and refrigerate if doing this in the morning.
Remove the hot baking sheet from the oven and spread the sweet potatoes on it in a single layer, then sprinkle with the salt.