Wash and halve the apples and cut out the core. Put the apples in an ovenproof dish, sprinkle with lemon juice, sprinkle with sugar and spread about 20 g fat in flakes on top.
Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 20-25 minutes. Meanwhile bring 500 ml water to the boil and pour into a bowl. Stir in the puree flakes with a mixing spoon and season with nutmeg.
After 1 minute, stir again briefly and allow to cool. Wash, clean and slice the mushrooms. Peel onions and cut into rings. Wash liver, dab dry and cut into small pieces.
Wash the thyme, dab dry and scrape the leaves off the stalks. Heat some fat in a pan and fry the onion rings until crispy brown. Put them on kitchen paper and let them drain.
Put the rest of the fat in the hot pan and fry the liver briefly. Add the mushrooms, fry briefly and season with salt, pepper and thyme, except for a little bit for sprinkling. Deglaze with stock and cream, bring to the boil and simmer for 3-4 minutes.
Add the sauce thickener while stirring and bring to the boil again briefly. Season with salt and pepper and keep warm. Put lukewarm mashed potatoes into a piping bag with a large star-shaped spout and spray a thick tuff on each baked apple.
Whisk the egg yolk and sprinkle the puree with it. Gratinate under the preheated grill for about 5 minutes. Arrange the liver ragout on plates and serve with onion rings and remaining thyme.