Melt the butter in a large pan and sauté the onions for 5 minutes at a low-medium heat until they are soft. Add the garlic, sage and potatoes, cover and let them cook at a very low heat for 10 minutes.
Stir in the stock and bring to a boil. After 5 minutes add the spinach some salt andn pepper and let it cook for 2-3 minutes. Remove the soup from heat and pass it through a food processor until it's smooth.
Serve hot with a spoonful of dairy cream and season with some extra sage.
