Preheat oven to 400 degrees F (200 degrees C). Place beets in a shallow pan.

Roast beets in the preheated oven until tender, about 1 hour. Remove pan from oven and cool beets until easily handled, at least 15 minutes.

Peel beets and cut into 1/4-inch-thick slices.

Beat balsamic vinegar, honey, and red wine vinegar together with a whisk in a bowl until dressing is smooth.

Mix beets and onion in a bowl. Drizzle dressing over beet mixture and toss to coat; refrigerate until chilled, about 1 hour.