In a glass bowl, combine carrots, onions, celery, and water. Microwave on high for five minutes, or until tender; set aside. For filling, combine egg, tuna, cottage cheese, lemon pepper, and half the cooked vegetables in a mixing bowl. Mix well. Spoon 1/3 cup mixture into each manicotti shell. Arrange filled shells in a 2-quart glass dish; set aside. In a saucepan, combine milk and flour. Cook and stir over medium heat until thickened and bubbly. Cook and stir one minute more. Add cheese, mix until melted. Stir in remaining cooked vegetables. Mix well. Pour over shells in dish. Cover with vented plastic wrap and microwave on high for eight minutes, or until heated through.