Cut the prepared leek into rings and the sausages into slices. Heat the fat. Briefly fry the sausages and leek and remove.
Sweat the flour in the frying fat. Pour in 500 ml of water and 375 ml of milk while stirring. Add the stock and peas, bring to the boil. Let simmer for 5 minutes.
Add the jacket potato with the sausage, leek and sour cream. Season the ragout well with salt, pepper and nutmeg. Garnish with chopped parsley.