Wash the meat, dab dry. Heat the fat in a pan and fry the meat over medium heat while turning for about 6 minutes. Peel onion, halve crosswise and cut into slices.
Wash apple, cut into quarters, remove core. Cut quarter into slices. Remove meat and keep warm. Sauté onion and apple in frying fat for about 5 minutes, add stock. Bring to the boil and simmer for 2-3 minutes, thicken with sauce thickener, bring to the boil again.
Boil 175 ml of water. Season with 1/4 teaspoon salt. Remove the pot from the heat, add the milk and stir in the puree flakes with a wooden spoon. Season sauce with sugar and mustard. Arrange puree, meat and sauce and garnish with parsley.