In a medium bowl, whisk together the mayonnaise, mustard, curry powder, salt and pepper. Add tuna, shallot, apple, walnuts, celery and pickle relish and toss until all ingredients are coated with dressing.
Lightly toast the croissants. Split in half, place a lettuce leaf on the bottom half of the croissant and fill with tuna salad. Top with a slice of Swiss cheese and the top half of the croissant. Serve with a dill pickle and potato chips. Bon appetit!