In a Dutch oven, sauté leek and celery with butter until tender-crisp. Add thyme, garlic and flour, stirring for another minute until the veggies are well coated.
Add broth, artichokes, and half of the lemon zest, and bring to a simmer cooking until all veggies are tender.
Add in the cream, spinach, and parsley and cook for another 3 minutes. Using an immersion blender or food processor, puree soup until smooth and return to pot.
Season to taste with salt and pepper, and stir in an of the remaining lemon zest and juice if needed to taste.
Notes
For low carb, reduce broth by 1/2 cup, and omit flour.