01 - Combine cooked chicken, celery, red onion, mayonnaise, Dijon mustard, dill, parsley, and lemon juice in a mixing bowl. Season generously with salt and black pepper. Mix thoroughly until all ingredients are evenly distributed and the mixture reaches a creamy, spreadable consistency.
02 - Using a vegetable peeler or mandoline, slice English cucumbers lengthwise into thin, wide ribbons. Discard the initial slice consisting primarily of skin. Continue slicing to create flexible ribbons that can roll without breaking but maintain enough structure to hold the filling.
03 - Pat cucumber ribbons thoroughly with paper towels to eliminate surface moisture. This critical step prevents the filling from becoming watery and ensures the roll ups maintain their structure during assembly.
04 - Lay a cucumber ribbon flat on a clean work surface. Spread approximately 1 tablespoon of the chicken salad mixture evenly along one short end. Top with a baby spinach leaf if desired. Starting from the filled end, gently roll the cucumber around the filling, creating a tight cylinder.
05 - Place completed rolls seam side down on a serving platter. Continue assembling with remaining ribbons and filling. Refrigerate for 10 minutes before serving to allow flavors to meld and improve texture.