Preheat the oven to 350 degrees F. Place the sweet potatoes into a large bowl. In a wok or saute pan, toast the cumin and Sichuan peppercorns for 1 to 2 minutes until fragrant. Transfer to mortar and pestle or spice grinder and grind. Sprinkle the spices and salt over the sweet potatoes, drizzle with oil and toss to coat. Transfer to a baking sheet and roast until tender, 30 minutes.;