Preheat oven to 375°.
Trim off beet stems and roots. Wrap each beet in foil; bake at 375° for 45 minutes or until tender. Cool beets to room temperature; peel and cut into 1/4-inch-thick slices. Combine beets, 4 tablespoons vinegar, 1/8 teaspoon salt, and 1/8 teaspoon pepper in a bowl; toss well.
Combine 1 tablespoon vinegar, 1/8 teaspoon salt, 1/8 teaspoon pepper, salad greens, and oil in a large bowl; toss well. Place 2 cups salad greens mixture on each of 4 plates; top with beets and onion rings.
