Combine ground beef, couscous, egg, mint, garlic, lemon peel and salt in medium bowl, mixing lightly but thoroughly. Shape into four 1/2-inch thick patties. Place patties in center of grid over medium, ash-covered coals. Grill, covered, 8 to 10 minutes (over medium heat on preheated gas grill, covered, 7 to 9 minutes) until instant-read thermometer inserted horizontally into center registers 160°F, turning occasionally.
Prepare Creamy Cucumber-Yogurt Sauce. Place burgers on bottoms of buns. Top with lettuce and onion, as desired. Spoon sauce evenly over lettuce and onion slices. Close sandwiches.
Nutrition information per serving: 402 Calories; 126 kcal Calories from fat; 14 g Total Fat; 4 g Saturated Fat; 4 g Monounsaturated Fat; 122 mg Cholesterol; 761 mg Sodium; 41 g Total Carbohydrate; 7.2 g Dietary Fiber; 31 g Protein; 4.4 mg Iron; 5.2 mg Niacin; 0.4 mg Vitamin B6; 2.3 mcg Vitamin B12; 5.8 mg Zinc; 27.9 mcg Selenium; 111.4 mg Choline;