In a small saucepan bring 1 cup water to a boil with 1 tablespoon of the oil and the salt, stir in the couscous, and let the mixture stand off the heat, covered, for 5 minutes. Stir in the remaining 2 tablespoons oil, the vinegar, the celery, the Parmesan, the scallion, the orange juice, the parsley, and salt and black pepper, to taste, and let the mixture cool to room temperature.
Press about 2 tablespoons of the couscous mixture in the palm of 1 hand, forming a log, and arrange it across a short end of 1 roast beef slice. Arrange 1 pepper strip along each side of the log and roll up the couscous and the bell pepper tightly in the roast beef. With a sharp knife cut the roll into 1-inch lengths, trimming the ends. Make rolls with the remaining roast beef, couscous mixture, and bell pepper in the same manner. (The rolls may be prepared 8 hours in advance and kept wrapped in plastic wrap and chilled.) Line a platter with the lettuce and arrange the rolls on top.