Place the cooked shrimp in a mixing bowl. Season with fish sauce, lime juice, garlic and cayenne. Mix well. Add lemon grass, red onion, green onions, and cilantro. Toss lightly and refrigerate. Chill a large platter. Rim it with lettuce leaves and lime slices. Sprinkle half of the mint in the center. Mound shrimp salad in the center and decorate with remaining mint leaves.