In medium bowl, whisk together orange juice, vinegar, maple syrup, salt and pepper. Set aside.
Place beets in 8-inch square baking dish. Bake in 375F oven about 1 hour, or until beets are tender (will depend on the size of the beets). Let cool. Peel and slice. Toss with 2 tablespoons of dressing.
In large bowl, gently toss watercress, oranges, fennel and walnuts. Divide over 4 plates. Top with beets and drizzle with remaining dressing.