Heat oven to 350 degrees F. Trim beets, leaving 1 inch of stem. Scrub skins. Place on aluminum foil and fold tightly to form packet. Bake at 350 degrees F for 30 to 35 minutes or until tender. Remove from oven and let cool. Slip skins off beets.
Heat oil in nonstick skillet; add onion and cook until wilted, about 5 minutes. Stir in vinegar, sugar and salt; cook 1 minute. Cut beets in half (or quarters if larger) and add to skillet; cook 2 more minutes. Remove from heat and place in serving bowl. Serve warm or at room temperature.