You can omit the step of roasting the onion, but roasting brings out a sweetness that complements the orange and contrasts with the watercress and endive. Preheat oven to 400F (200C) Cut onion in half and slice each half into 4 wedges. Separate wedges into individual pieces. In a bowl, toss onions with olive oil and balsamic vinegar. Season with salt and pepper. Place on cookie sheet, bake for 20 minutes or until onion wedges are browned. Reserve. Clean stalks from watercress and toss leaves with onions. Slice Belgian endives into 1/2-inch slices and add to bowl. Wash and stem spinach. Dry well and tear into bite-sized pieces. Cut skin and white pith from oranges. Over a bowl, cut oranges into segments, reserving the juice that escapes. To make dressing, combine lemon juice with reserved orange juice, add Dijon mustard and wine vinegar, then whisk in olive oil. Season with salt and pepper. Just before serving, toss lemon vinaigrette with onions, orange and vegetables.