Preheat the oven to 180°C.
Place cut sweet potatoes into a large bowl.
In a wok, or saute pan, toast the cumin and Sichuan peppercorns for one to two minutes until fragrant. Transfer to mortar and pestle or spice grinder and grind.
Sprinkle spices and salt over the sweet potatoes and then drizzle with oil and toss to coat. Transfer to a baking sheet and roast in the oven for 30 minutes, or until tender. Perfect with Golden Gate Chilli Ribs.