Preheat oven to 375°F. Trim ends from beets (do not peel); cut into 1-inch wedges or chunks. Trim onion and cut into 1-inch chunks.
Combine olive oil and seasoning in a bowl. Add beets and onions; toss to coat.
Place seasoned vegetables on a large sheet of foil in a single layer. Fold foil over vegetables and seal, forming a package. Place on a baking sheet. Bake for 1 1/2 hours. Remove from oven and carefully open packet (avoiding steam). Serve immediately.