Preheat the oven to 375°F. Cut 2 pieces of aluminum foil, each 10 × 12 inches, and place one on top of the other. Place the beets in the centre of the foil and add the oil, thyme, garlic and salt. Fold the foil around the beets and roll up the edges to create a sealed package. Set the beets on a baking tray and roast for 30 to 40 minutes. To check for doneness, remove the beets from the oven, carefully unwrap them and insert the tip of a knife into one of them. The knife should slide easily in and out.