Make gremolata: In a small bowl, combine the parsley, lemon zest and garlic, Stir to mix and set aside.
Heat a large, heavy-bottomed soup pot over medium heat. Add the oil to the pot. Add the onions and celery to the pot. Saute the onions and celery until very soft and slightly browned, about 6 minutes. Add the garlic, coriander, cumin, salt and pepper to the pot and stir. Keep cooking the vegetables and spices for another minute.
Add the potatoes and lentils to the pot and stir. Season again with salt and pepper. Add the vegetable stock to the pot and stir, scraping up any browned bits on the bottom. Cover the pot and bring the soup to a boil. Once boiling, lower the heat to a simmer. Simmer stew with the lid on for 25 minutes, or until the lentils and potatoes are both tender.
Add the spinach, parsley, and basil to the pot. Stir until the spinach just starts to wilt, about 30 seconds. Then, carefully ladle half of the stew into an upright, vented blender. Add the tahini to the stew in the blender. Close the lid on the blender and slowly bring up the speed to high. Blend until stew is totally smooth. If you have to add a bit of water to get it moving, definitely do so.
Scrape the blended stew back into the pot and bring the now unified stew up to a boil. Squeeze in the lemon juice, add the tamari, stir, and then taste the stew. Add more stock (or water) at this point if it seems too thick. Adjust salt and pepper if necessary. I find that this stew needs quite a bit of salt. Serve the deep green lentil stew hot with the gremolata on top, swoops of olive oil, the chili flakes, extra lemon, and extra chopped herbs.