2) Meanwhile, melt half the butter in a pan over a low heat and add the onions. Cover and cook very gently for 15 minutes, until soft and translucent.
3) Drain the potatoes thoroughly and allow to cool slightly before peeling. Mash and stir in the remaining butter and enough milk to make the potatoes smooth and creamy.
4) Turn the heat up on the onions to medium and add stir in the sugar. Stir well and allow to cook for two to three minutes, until golden and caramelised. Stir into the potatoes and season well with salt and pepper.