Using the whisk on a hand mixer, beat the eggs, sugar and salt for approx. 5 mins. until the mixture is light and fluffy. Stir in the crème fraîche. Mix the flour, hazelnuts, oats and baking powder, stir into the egg mixture. Add the diced apple and lemon juice, mix, transfer to the prepared cups.
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. Leave to cool on a rack.