Add water, onion, celery, garlic, salt and pepper in a skillet over medium-high heat. Cook 9-10 minutes, stirring occasionally, until softened and turning golden. In a food processor, add chickpeas (except reserved 1/3 cup), lemon juice, tamari, sage, salt, and sautéed mixture, and partially puree (not fully like hummus, but leaving some chunkier consistency). Add toasted walnuts and oats, and briefly pulse to lightly break up nuts. Transfer to a bowl, and stir in spinach, cranberries, parsley, thyme, and reserved chickpeas. Transfer mixture to pie shell (or lightly oiled pie plate, see note), smoothing to evenly distribute. Brush tamari over top. Sprinkle on walnuts. Bake in preheated oven at 400 degrees for 30-35 minutes, until tart is golden on edges and top. Cool 5-10 minutes, then serve with cranberry sauce, a spoon drizzling of oil/balsamic vinegar slurry, or other sauce of choice. Serves 4-5.