Whisk together all ingredients. Cover and chill 30 minutes before serving. Store in an airtight container in refrigerator up to 3 days.
To Prepare the Shrimp:
Toss shrimp with 1 tablespoon olive oil in a medium bowl. Thread shrimp on skewers and sprinkle both sides with Cajun or Creole seasoning.
Cut romaine lettuce hearts in half lengthwise. Brush cut sides of hearts with remaining 1 1/2 tablespoons olive oil.
Preheat oven to 375 degrees. Cut baguette into 18 (1/2-inch-thick) slices; place on a baking sheet. Stir together butter and garlic. Brush 1 side of bread with butter mixture. Bake 8 to 10 minutes or until golden.
Meanwhile, preheat grill to 350 - 400 degrees (medium-high) heat. Grill shrimp, covered with grill lid, 3 to 4 minutes on each side or just until shrimp turn pink. Remove shrimp, and cover with aluminum foil to keep warm. Place romaine, cut sides down, on cooking grate of grill. Grill, covered with grill lid, 2 to 3 minutes or until just wilted and grill marks appear. Grilling is optional, otherwise proceed to next step.
Place 1 grilled romaine heart half on each of 6 plates. Top with tomato wedges, onion, and shrimp. Drizzle with Remoulade Dressing. Serve with toasted French bread slices.