Peel, wash and halve the potatoes and cook in boiling salted water for about 20 minutes. Add the peas 5 minutes before the end of the cooking time. In the meantime peel onion and cut into rings.
Wash the apple, quarter it, cut out the core. Cut apple into slices. Wash liver and dab dry with kitchen paper. Heat oil in a pan. Fry the onion and apple until golden brown, remove from the pan and set aside.
Place the liver in the cooking fat and fry for approx. 4 minutes, turning. 1 minute before the end of the frying time, add the apple and onion to the liver again and season with salt and pepper. In the meantime heat milk and fat in a pot.
Drain potatoes and peas, add milk and mash with a potato masher. Season to taste with salt, pepper and nutmeg. Arrange potato and pea puree and liver with apple and onion on a plate and serve garnished with basil as desired.