1. In a small saucepan, heat 2 tablespoons of the oil, add the onion, and cook over low heat for 3 minutes. Stir in the curry powder and salt, and cook another 30 seconds. Remove the mixture from the heat and let it cool. Stir in the mayonnaise, lemon juice, and mustard.
2. Put the chicken into a serving bowl and add the curry-mayonnaise mixture, the remaining 2 tablespoons of oil, and the celery. Toss to mix. Add the grapes and toss again, gently. Chill the salad before serving if there is time. If time is short, start with very cold chicken and grapes.