In a large skillet, heat olive oil over medium heat. Add chopped chicken and cook until chicken is cooked through. Remove from heat and set aside to cool.
In a large bowl, stir together the celery, walnuts, cranberries, onion, mayonnaise, vinegar, salt and pepper and lemon juice. Add cooled chicken, and stir until well combined. Cover and chill about 6 hours and serve on top of spinach leaves, if desired.