Place chopped onions and sweet potatoes in a single layer on a rimmed sheet pan. Drizzle with olive oil, spices and salt and pepper. Toss to evenly distribute oil and spices over the vegetables. Place in a 425 degree oven for 15 minutes, then toss veggies with a spatula and return to oven for another 10-15 minute, until veggies are cooked through and starting to caramelize.