Heat oven to 425°F. Remove greens from beets, leaving about 1/2 inch of stem. Do not trim or cut root. Wash beets well. Tightly wrap beets in double layer of foil, and roast on cookie sheet 1 to 1 1/4 hours or until tender. When beets are cool enough to handle, peel them, discarding stems and root ends, then cut into 1/2-inch-wide wedges.
In small bowl, mix dressing ingredients with wire whisk.
In large bowl, toss half the dressing with mixed greens. Divide evenly among 4 salad plates. Arrange beets, nectarines, cheese and walnuts over each salad. Drizzle remaining dressing over each salad. Serve immediately.