Coat the bottom of a large, heavy-bottomed pot with 1/4 inch of olive oil. Cook celery on medium heat until it starts to soften, 5-6 minutes. Add the salt, pepper, minced and granulated onion, celery salt and celery seed. Continue cooking until celery is very soft, another 7-8 minutes. Then add the diced onions. Saute until the vegetables are browned, another 8-10 minutes.
Add the vegetable stock and stir to combine all ingredients. Cover and simmer on low heat for at least 1 hour.
Off heat, add the spinach and puree with an immersion blender. Alternately, allow the soup to cool, then transfer to a standard blender, add the spinach and puree until it reaches the desired consistency.