Place a rack in the center of the oven and heat to 425 degrees. Wrap the beets individually in foil, sealing the edges. Bake until they are easily pierced with a tip of a knife. 35 minutes to 1 hour depending on their size. When cool enough to handle slip off the skins and cut the beets into 1/2 inch cubes. Wash the dandelion leaves well to remove all sand and grit. Spin them dry and put in a large serving bowl. Add the beets and walnuts. In a small bowl, combine the vinegar, salt, pepper and chives. Slowly whisk in the oil until well-combined. Drizzle the vinaigrette over the salad, toss well and serve.