01 - Combine chicken, celery, red onion, mayonnaise, Dijon mustard, dill, parsley, lemon juice, salt, and pepper in a mixing bowl. Mix thoroughly until all ingredients are evenly distributed.
02 - Using a vegetable peeler or mandoline, slice cucumbers lengthwise into thin, wide ribbons. Discard the first slice which is mostly skin. Create ribbons that are flexible yet substantial enough to hold filling.
03 - Pat cucumber ribbons thoroughly with paper towels to remove excess moisture. This step ensures the roll ups maintain structure and don't become watery.
04 - Lay a cucumber ribbon flat on a clean work surface. Spread approximately 1 tablespoon of chicken salad mixture along one end. Place a spinach leaf on top if desired. Gently roll the cucumber around the filling, starting from the filled end.
05 - Place completed roll ups seam side down on a serving platter. Continue until all ribbons and filling are used. Refrigerate for 10 minutes before serving to allow flavors to meld and improve texture.