Preheat oven to 425 degrees F.Combine the sweet potatoes, carrots and onions in a large bowl. Drizzle with the olive oil, adding more if necessary to lightly coat, sprinkle with the seasonings and salt and pepper. Toss to coat.Spread into an even layer on a lightly greased baking sheet.Roast for 25-30 minutes, turning halfway through. Vegetables should be soft and lightly browned when done.Season with additional salt and pepper if desired. Serve hot.