Place beets in a 13-in. x 9-in. baking dish; add 1 in. of water. Cover and bake at 400° for 40-45 minutes or until tender. Cool; peel and julienne., In a serving bowl, combine the lettuce, oranges, endive and watercress. Add beets and walnuts. , In a small bowl, whisk the oil, orange juice, vinegar, onion, lemon juice, mustard, salt and pepper. Drizzle over salad; toss gently to coat.