Wash the liver and pat dry. Clean, wash and cut the leek into rings. Cook the pasta in plenty of salted water for about 12 minutes. Drain on a sieve. In the meantime heat 3 tablespoons of fat in a pan.
Brown the liver on both sides. Add chicken soup and bring to the boil. Cook over medium heat for 5 minutes. Add leek and cook for another 5 minutes. Add sour cream and brandy.
Knead flour and remaining fat, add to the sauce while stirring and bring to the boil once. Season to taste with salt and pepper. Arrange the ragout with the spaghetti in a preheated bowl.