Peel the apples and potatoes, wash the potatoes, grate both finely and mix together. Wash the marjoram and thyme leaves, pat dry, chop finely and stir into the apple-potato mixture with lemon juice, egg yolk and potato starch. Season the mixture with salt and pepper.
Whisk the eggs. Turn the mozzarella slices first in flour, then in beaten egg and then coat with the potato mixture. Fry in the heated oil for about 10 minutes until crispy and golden brown.