First off, toss your veggies in an oven proof dish, drizzle with olive oil and season with salt and black pepper. Grill / roast your butternut and sweet potato for about 20 minutes on 200 degrees Celsius. Check it regularly and if you want, you can cover your veggies with an ovenproof lid or tin foil – that helps with the cooking process, for the first half of the cooking time.