Preheat oven to 400 degrees. Heat oil in a skillet over medium heat. Add onion, celery, garlic, salt, and pepper, and cook for 7 to 9 minutes until onions begin to soften and caramelize. Set aside.
In a food processor, add chickpeas (except reserved 1/4 cup), walnuts, lemon juice, and tamari, and pulse to lightly chop (do not puree). Remove half of chopped mixture from food processor and transfer to a large bowl.
Transfer sauted onions to the remaining mixture in food processor. Puree until smooth, scraping down sides of bowl as needed.
Transfer puree to bowl with lightly chopped chickpea and walnut mixture. Add spinach, cranberries, parsley, thyme, and 1/4 cup of reserved chickpeas. Stir to combine and transfer to prepared pie shell, spreading evenly.
Combine oil and tamari, and brush on top of pie and crust. Sprinkle with walnuts. Bake for 33 to 38 minutes until lightly browned. Garnish with parsley, cut in wedges, and serve with cranberry sauce.