Beat the fat, sugar and eggs with the whisk of the hand mixer until foamy. Add salt, lemon zest and juice. Mix flour and yeast. Add the quark to the bowl. Knead everything into a smooth dough with the dough hooks of the hand mixer.
Cover and leave to rise in a warm place for about 30 minutes. About 10 minutes before the end of the rising time, divide the white cooking fat into coarse pieces and heat them up to about 190 °C in a closed deep fryer. In the meantime, wash, peel and quarter apples and cut out the core.
Finely dice apples and fold into the dough. As soon as the frying temperature is reached, use 2 tablespoons to cut off small dumplings. Do not bake more than 3-4 at a time in hot fat, because the pastry will still rise and must be able to swim freely.
Fry for about 5 minutes until golden brown. Remove the doughnuts with a skimmer and drain well. Roll the hot doughnuts in the sugar. After each frying process wait until the fat has reached the desired temperature again.