Tuna Salad Croissant Sandwich

Name: Tuna Salad Croissant Sandwich
Ingredients: 2 cans tuna packed in olive oil (drained); ½ cup celery (diced, 1-2 celery stalks); 1 small shallot (minced); 2 tbsp fresh dill (chopped); 2 tsp capers; 1 tsp lemon zest; 1 tbsp freshly squeezed lemon juice; 1 tsp dijon mustard (or to taste); ½ cup kewpie mayo (or regular mayo); kosher salt (to taste); freshly ground black pepper (to taste); 4-8 butter or romaine lettuce leaves; 4 croissants (sliced in half lengthwise); 1-2 tomatoes (thinly sliced); 1 cup sprouts (alfalfa, broccoli, radish, or a mix); cornichons (optional)