Preheat oven to 425 degrees. Cut the tops off the beets leaving about 2 inches of the stalk. Scrub beets well and wrap in foil with 2 beets of equal size in each packet.
Place on a baking sheet and bake in preheated oven until beets yield to gentle pressure; approximately 1 hour. Remove from oven, unwrap and allow to cool. When cool enough to handle, slip the skins off of the beets; set aside.
While the beets are roasting, place the maple syrup through lemon juice in a blender container. Blend until well combined. With blender running, drizzle in the olive oil and continue to blend until thickened.
To assemble the salad, place the greens in a large bowl and drizzle with a small amount of dressing, toss to coat. Divide the greens among 6 chilled plates. Quarter beets or if large cut in eights and divide among the plates, top with onion rings, sprinkle feta and walnuts over all.