In a bowl, toss the beets with 1/2 tablespoon olive oil to coat. Spread in a single layer on a baking sheet.
Mix the remaining 2 tablespoons olive oil, garlic powder, salt, pepper, and sugar in a large resealable plastic bag. Place the sweet potatoes and onion in the bag. Seal bag, and shake to coat vegetables with the oil mixture.
Bake beets 30 minutes in the preheated oven. Mix sweet potato mixture with the beets on the baking sheet. Continue baking 45 minutes, stirring after 20 minutes, until all vegetables are tender.