In a large saucepan, over medium high heat, heat the oil with the onions. Sear for 5 minutes until the onions start to colour.
Add the leeks, garlic, sage and potatoes, stirring them all round with a wooden spoon for a minute or so. Season with salt, pepper and some chilli flakes.
Add the stock or water and the cream and bring to boil. Drop the heat to medium/low, cover and let the soup simmer very gently for a further 10-15 minutes or until the vegetables are soft.
remove the soup from the heat and using a hand blender, blend the soup until smooth.
Check the seasoning. Add the butter and swirl to enrich the soup. Keep warm.
To serve pour the soup into bowl and sprinkle with extra pepper, chilli flakes and a drizzle of cream.