01 - Combine cooked 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 and mixture is creamy and well combined.
02 - Using a vegetable peeler or mandoline, slice cucumbers lengthwise into thin, wide ribbons. Discard the first slice which is mostly skin. Continue slicing to create flexible ribbons that can roll without breaking, approximately 1/8 inch thick.
03 - Pat cucumber ribbons dry with paper towels to remove surface moisture. This step prevents the roll ups from becoming watery and helps the filling adhere better to the cucumber.
04 - Place a cucumber ribbon flat on a clean work surface. Spread approximately 1 tablespoon of chicken salad mixture along one short end of the ribbon. Add a spinach leaf on top of the filling if desired. Gently roll the cucumber around the filling, creating a tight cylinder.
05 - Place completed roll ups seam side down on a serving platter. Repeat with remaining cucumber ribbons and filling. Refrigerate for 10 minutes before serving to allow flavors to meld and roll ups to set.