1. Combine the lime juice, cilantro, scallion (saving some for garnish), hoisin sauce, oil, garlic, and white pepper in a large bowl. Whisk to mix. Set aside.
2. Warm 1 tablespoon of the reserved mixture in a large nonstick skillet over medium heat. Add the shrimp. Cook, tossing, for 2 to 3 minutes, or until the shrimp are opaque. Pour the skillet contents into the reserved mixture. Add the bell pepper, and avocado. Cover and refrigerate, tossing occasionally, for 30 minutes. Place four serving dishes in the refrigerator, if desired.
3. Line each chilled plate with three lettuce leaves and fill with ⅓ cup of the marinated shrimp mixture. Garnish with sliced scallions and serve.