1. Cut baguette in half horizontally. Hollow out top and bottom halves of bread, leaving a 1/2-inch-thick shell; reserve torn bread for another use. Combine mayonnaise and next 5 ingredients (through garlic) in a small bowl; stir well. Spread mayonnaise mixture evenly over cut sides of bread.
2. Combine cornstarch and cornmeal in a shallow dish. Dredge shrimp in cornmeal mixture; shake off excess cornmeal mixture.
3. Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add shrimp; cook 3 minutes on each side or until done.
4. Arrange lettuce, shrimp, and onion on bottom half of baguette; cover with top half of baguette. Cut loaf into 4 pieces. Serve immediately.