Heat up a large saucepan or deep frying pan to medium heat. Melt the butter and then add the chopped onions and fry until soft and translucent, roughly 5 to 6 minutes.
Add the crushed garlic to the pan and fry for a minute or two longer until it is fragrant.
Add the diced potatoes to the pan. Then, deglaze the pan by adding in the stock. Gently simmer the mixture until the potatoes are cooked through.
Add the spinach to the pan and cook for 2 to 3 minutes until it is wilted.
Transfer the soup to a food processor and blend until it is smooth.
Return the blended soup to the saucepan and add the cream, a pinch of nutmeg, and salt and pepper to taste.
Serve either hot or cold. Finish off with basil oil, extra cream, and chopped chives if desired.