In a saucepan combine the broth, the water, 1/4 cup of the lemon juice, and 2 tablespoons of the oil, bring the mixture to a boil, and stir in the couscous. Cover the pan, remove it from the heat, and let the couscous stand for 5 minutes. Fluff the couscous with a fork and let it cool in the pan. In a very large bowl stir together the cucumber pieces, the tomato dice, the scallion, the remaining 1/3 cup oil, the remaining 1/4 cup lemon juice, and salt to taste and let the mixture stand for 15 minutes. Add the couscous, the parsley, and the mint leaves, stir the salad well, and chill it, covered, for 1 hour. The salad may be made 2 days in advance and kept covered and chilled. Serve the salad garnished with the mint sprigs and the cucumber and tomato slices. Serves 8 to 10. Gourmet June 1992