Heat the olive oil in a medium pot over medium heat.
Add the garlic and onion, cooking until fragrant and softened.
Stir in the carrot and celery, cooking for a few minutes until slightly tender.
Add the potatoes, stock, and bay leaf. Cover and bring to a boil.
Reduce the heat and let simmer until the vegetables are softened.
Remove the bay leaf, then scoop out about one-third of the vegetables and set them aside.
Stir in the double cream and chilled butter, mixing until the butter melts completely.
Use a stick blender to puree the soup until smooth. If using a stand blender, let the soup cool slightly before blending.
Season with salt, pepper, and lemon zest. If using cumin or nutmeg, add them now and mix well.
Stir in the spring onions, taste, and adjust seasoning if needed.
Return the reserved vegetables to the pot, cover, and let simmer for another three minutes.
Serve hot, garnished with extra spring onions if desired. Enjoy!