In a large mixing bowl, combine the cooked shrimp, mayonnaise, Dijon mustard, lemon juice, celery, red onion, and dill. Mix well until all ingredients are evenly coated.
Season the shrimp mixture with salt and pepper to taste. Adjust the seasoning as desired.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavors to meld.
Once chilled, toast the bread slices if desired.
To assemble the sandwich, place a generous scoop of the shrimp salad on two slices of bread. Top with lettuce leaves and tomato slices if using. Place the remaining bread slices on top to complete the sandwiches.
Cut the sandwiches in half and serve immediately.