Heat oil in a saucepan over medium heat. Add celery & onion and cook for about 5 minutes.
Combine everything but the spinach & lemon juice and simmer for about 30 minutes, then remove the bay leaf.
Optional: puree a few spoonfuls of the soup in a blender, and then add it back into the pot.
Stir in spinach and cook for another minute, then remove from heat and stir in lemon juice.