Make tuna salad - Add the tuna to a large mixing bowl and use a fork to break up the fish into small flaky pieces. Add the diced celery, minced shallot, chopped dill, capers, lemon zest and juice, dijon mustard, and mayonnaise to the tuna. Fold the ingredients together until combined. Season with salt and pepper to taste.
Assemble sandwich - Lay the croissant bottoms cut side up on a serving plate and top with 1-2 slices of lettuce and sliced tomatoes. Layer a scoop of tuna salad on top of the tomatoes and spread it out evenly, then sprinkle micro-greens on top. Place the croissant tops on each sandwich and gently press down. If desired, put a couple of cornichons on toothpicks and place through the top of the sandwich to hold it in place. Serve and enjoy!
