Melt butter in a pan over low heat.
Add the sliced leeks and a big pinch of salt and cook for 15 minutes until soft, without browning.
In a separate pan, add chicken stock, milk and oats.
Cook on low for about 30 minutes, stirring regularly.
Blend the oat mixture until smooth, then strain through a fine sieve.
Stir in double cream then pour the oat mixture over the cooked leeks.
Simmer for 10-15 minutes, season with salt and white pepper to taste.
Serve with chopped parlsey on top.
