Peel shrimp; devein, if desired. Toss shrimp with 1 Tbsp. olive oil in a medium bowl. Thread shrimp on skewers, and sprinkle both sides with Cajun seasoning.
Cut romaine lettuce hearts in half lengthwise. Brush cut sides of hearts with remaining 1 1/2 Tbsp. olive oil.
Preheat oven to 375°. 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 350º to 400º (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.
Place 1 grilled romaine heart half on each of 6 plates. Top with tomato wedges, onion, and shrimp. Drizzle with Rémoulade Dressing. Serve with toasted French bread slices.