Wash the leeks thoroughly, chop into rings. Using a knife, roughly crush the oat flakes.
In a big pot, melt butter, add the oatmeal and fry on medium heat for about 5 minutes, stirring until golden brown.
Still stirring, pour in the broth and milk. Add the chopped leek, salt, pepper and nutmeg, and bring to a boil. Lower heat and simmer for 40 minutes, partially covered, stirring regularly.
Blend the soup using a hand blender, season to salt and pepper and serve with fresh parsley and an optional splash of Marmite!