*Note: Shrimp should be shelled and deveined. Combine lime juice, 3 tablespoons green onion, salt and pepper in medium bowl. Mix in shrimp. Cover and refrigerate until shrimp turn opaque, stirring occasionally, at least 6 hours. (Can be prepared 1 day ahead.) Drain shrimp thoroughly, discarding marinade. Fold in creme fraiche and capers. Arrange lettuce on plates or scallop shells. Top with shrimp. Garnish with lime slices and 1 tablespoon green onion. Serve immediately. CREME FRAICHE: Combine cream and buttermilk in small jar. Cover tightly and shake well for 1 minute. Let stand at room temperature until thickened, about 8 hours. Store in refrigerator.