Melt all the butter into a pan add the chopped leeks, cover and cook for about 10 minutes until soft.
Take leeks off the heat and add the white sauce mix and milk and mix.
Return to the heat and bring to boil for 3-4 minutes.
Add cheese and puree with a blender to desired consistency.
Serve with croutons.
A mushroom soup is a great way to use up that big punnet that seemed like a bargain! This leek and mushroom soup is particularly rich and creamy and uses one of those other ingredients that often lingers in the cupboard - white sauce mix. If you don't have any then creme fraiche or cream are a great substitute and can be thickened up with some flour.