To make the filling attach the paddle attachment to the food processor. Alternatively you can use a hand mixer.
Mix the quark and the cream cheese until very fluffy. Continue on the lowest speed and add the sugar, flour, vanilla extract, and salt.
Add eggs: Mix to incorporate, then start adding the eggs and the egg yolk, one by one, mixing well in between. Pour in the heavy cream and combine.
Peel the apples and chop them into small cubes. Mix with 2 tablespoons lemon juice.
Brush the walls of the springform with some butter. The form is hot so that the butter will melt, and brushing will be very easy.
Fill: Place the chopped apples on the base of the cake and carefully pour in the filling. Stir gently to make sure that the apples don't get pushed towards the walls of the springform. Sprinkle the cake with the streusel.
Bake for 40 to 50 minutes. The streusel should be pretty brown, and the middle of the cake is still a bit wobbly.
Let rest in the oven: Turn off the heat but leave the cake inside the oven. Place a wooden spoon in the oven door to prevent it from closing and leave the cake for another two hours in the oven.
Cool: Take it out and let it cool completely. Best served the next day.