Peel, wash and quarter the potatoes. Cook in boiling salted water for about 20 minutes. In the meantime, segregate the spinach, wash it and let it drip off. Peel onion and garlic. Cut onion into slices, press garlic through a garlic press.
Wash the fillet and dab dry. Heat the oil in a pan. Fry the fillet for about 10 minutes. Season with salt and pepper. Peel mint leaves and cut into strips. Add garlic and mint to the frying fat, drizzle over the fillets and wrap in aluminium foil.
Fry the onion slices in the frying fat. Add the spinach and let it collapse completely. Season with salt and pepper. Drain the potatoes. Add yoghurt and mash to puree. Season to taste with salt and pepper.
Cut the fillet open. Serve with puree and spinach.