Wasabi is the Japanese version of horseradish. It comes in powder form and is available in Asian groceries and some supermarkets. It's green and fiery. If you prefer, though, you can substitute prepared horseradish.
1 tablespoon prepared wasabi powder (dried Japanese horseradish)
Place first 8 ingredients in a blender; process until smooth. Set aside.
Leave root and 1 inch of stem on beets; scrub with a brush. Place beets and sweet potato in a shallow baking dish. Bake at 400° for 45 minutes. Add mushrooms, and bake an additional 15 minutes. Cool to touch. Peel beets and potato; cut into 1/2-inch cubes. Thinly slice mushrooms.
Combine beets, potato, mushrooms, greens, and chicken. Drizzle dressing over salad; toss well. Serve immediately.