Heat the butter and oil in a large frying pan and cook the onions very slowly until soft and golden. Add the potato slices and garlic and stir to coat with the buttery onions. Season well with salt and cover with a lid.
Leave to cook over the lowest flame, stirring occasionally then recovering. They will take about 45 minutes and should be soft inside and caramalised on the outside.
Stir through the peppercorns and a handful of fresh dill before serving.