Clean all your vegetables and grab a large soup pan. Heat some olive oil and bake the onion and garlic. Add the celeriac, the celery and the parsnip to it and stir together. Fry for a bit and then add the stock to it, bring to the boil and turn the heat down.
Leave to simmer for about 30 minutes or until the vegetables are cooked. Add the spinach the last five minutes.
Puree the soup with an immersion blender.
Add the cream if using and stir in. Taste if you need to add any pepper and salt.