Mix about 1 l water and lemon juice in a bowl. Wash, peel, quarter and core the quinces. Cut the flesh into thin slices and put them immediately into the lemon water to prevent them from turning brown. Cut open the vanilla pod and scrape out the pulp. Cut the pod into small pieces.
Heat butter in a pot. Drain the quinces a little, fry them with vanilla pulp and vanilla pod in the hot butter. Sprinkle sugar on top and caramelise briefly. Deglaze with apple juice and stew covered for about 25 minutes.
Wash the fillets, dab dry and cut the fat layer several times, but do not cut through. Season fillets on the meat side with salt and pepper. Heat a pan without fat. First fry the fillets on the skin side for approx. 6 minutes, then turn them over and continue frying for 1-2 minutes on the meat side. Place the fillets on a baking tray and continue to cook in a preheated oven (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer) for approx. 8 minutes. Remove the meat from the oven and let it rest for a short time wrapped in foil.
Stir starch and 2 tablespoons of water until smooth. Stir into the quinces, bring to the boil again and simmer for about 3 minutes. Season to taste with salt and pepper. Cut the fillets into slices and arrange with the quinces. Goes well with spaetzle tossed in hazelnut butter.