Heat up olive oil on a medium-sized pot.
Once olive oil is ready, add garlic and onion.
Cook until garlic becomes fragrant and onion becomes softer.
Add carrot and celery. Mix and cook for a few minutes.
Add potatoes, stock and bay leaf.
Cover with a lid and bring to a boil.
Keep simmering until the vegetables are slightly cooked (i.e. softened).
Remove the bay leaf. Scoop about 1/3 of the vegetables and set aside.
Add double cream and butter. Mix until butter has completely dissolve.
Blitz the soup by using a stick blender. (Alternatively, you can use a stand blender, but you have to make sure the soup has cooled down a bit.)
Season with salt and pepper. Add lemon zest. (Although cumin and nutmeg are optional, you should add them now if you intend to use them.)
Mix to combine everything.
Add spring onions. Mix, taste and adjust the seasoning, if desired.
Add back the 1/3 of the vegetables that was set aside earlier. Cover and simmer for about 3 minutes.
Serve. Garnish with more spring onions, if desired.