Preheat the oven to 375°F.

Wrap beets individually in aluminum foil and place on a rimmed baking sheet.

Roast until beets are tender when pressed through the foil and a knife slides easily into them when unwrapped, 50 to 60 minutes.

When cool enough to handle, unwrap beets and rub each with a paper towel or dark kitchen towel to remove skins.

Halve and slice beets.

Meanwhile, in a large baking dish, combine sweet potatoes, red potatoes and onion with oil, thyme, rosemary, salt and pepper.

Roast for about 1 hour, or until soft when pierced with a knife, stopping halfway through to gently stir.

Arrange all roasted vegetables together on a serving platter or bowl. Serve warm.