Place the crème fraîche into a bowl. Rinse and dry the crème fraîche pot, use as a measuring cup. Using the whisk on a hand mixer, beat the eggs, sugar and salt for approx. 5 mins. until the mixture is light and fluffy.
Mix the flour, hazelnuts, oats and baking powder, stir into the egg mixture. Cut the apple into small cubes, set aside 3 tbsp. Add the remaining diced apple and lemon juice, mix into the dough.
Grease the cups with butter and dust with flour. Line the cups with dough.
Melt the butter and allow to cool slightly. Mix the oats, sugar and butter with the reserved diced apple, scatter on top of the cakes.
Approx. 40 mins. in the centre of an oven preheated to 180°C. Remove from the oven, leave to cool on a cooling rack.