Make the vinaigrette: whisk together the lemon juice, olive oil, salt and pepper. Stir in the onion. Cook the corn in boiling salted water for 5 minutes. Drain. In a salad bowl combine the chicken, rice, corn, celery, fennel, watercress, tomatoes, curly endive and vinaigrette., Toss to mix and stand for 1 hour. Make the mayonnaise: In a small bowl beat the egg yolks until thick with a little salt, pepper, the mustard, and half the vinegar. Add the oil, drop by drop, whisking constantly until the mixture thickens. Once the mixture is thick, pour the remaining oil from a jug in a very slow stream, whisking constantly. Stir in the rest of the vinegar. Drain the vinaigrette from the salad and transfer the salad to a serving platter. Mix the vinaigrette from the salad with 2 tablespoons of the mayonnaise and the olives. Serve the salad with the remaining mayonnaise and the olive mayonnaise on the side.
