Cook the potatoes in a little water until tender. Meanwhile remove the core from the apples, peel them and cut in half. Cook gently in a pan with 2 oz (50 g) of the butter, the sugar and a little water, until they are just soft and retain their shape. The water will evaporate leaving a buttery sauce.
Drain the potatoes and make a purée with the warm milk and 2 oz (50 g) of the butter. Season. Heat the oil and fry the onion. Remove from the pan and keep warm. Fry the sausages. On a warmed serving dish, first put the potato purée, then cover with the onion and surround with the sausages and warm apples.