Roast the nuts in a pan without fat while turning them golden brown. Let them cool down on a flat plate. Cream mascarpone, butter, 1 egg, starch and 2 tbsp. sugar with the whisk of the hand mixer.
Set aside. Peel and quarter apples, remove core and cut quarters into small cubes. Drizzle apple cubes with lemon juice and set aside.
Finely chop 100 g nuts. Set the rest aside for decorating. Mix 3 eggs, oil, brown sugar, salt, vanillin sugar and applesauce with the whisk of the hand mixer. Mix flour, cinnamon and baking powder.
Stir the flour mixture into the egg mixture. Fold in diced apples and chopped nuts.
Fill about half of the dough into a greased, flour-blasted tube bottom mould (2.8 litres capacity) and smooth it down. Spread the mascarpone mixture in the middle of the dough so that there is a distance of approx. 0.5 cm between the dough and the cake tin.
Add the rest of the dough and smooth it down. Bake the cake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 1-1 1/4 hours. Remove from the oven and leave to cool in the mould for about 15 minutes.
Drop onto a cake rack and let it cool down.
Meanwhile caramelise 75 g sugar in a small pot until golden brown. Add cream and simmer over low heat, stirring occasionally, until the caramel has dissolved. Let the sauce cool down.
Arrange the cake on a plate. Roughly chop the nuts. Decorate cake with sauce and nuts.