Heat a skillet over medium-high heat and add the shrimp. Sprinkle Cajun seasoning over them and sauté for 2-3 minutes on each side until pink and opaque.

Chop celery and red onion into small, even pieces. Add the chopped vegetables to a mixing bowl.

Add the cooked shrimp to the bowl with vegetables. Incorporate mayonnaise, Dijon mustard, and lemon juice.

Taste the mixture and adjust seasoning with salt and pepper as needed.

Layer lettuce leaves on one slice of bread, fill with shrimp salad mixture, and top with another slice of bread.

Cut the sandwich in half and serve immediately for best flavors.