Cook the potatoes in boiling salted water for 15mins or until tender. Meanwhile, place the sausages on a grill rack and cook for 10-12mins, turning occasionally until golden on all sides.
Whilst the potatoes and sausages cook, heat the oil in a medium pan, add the onions and sauté for 8 mins or until golden. Stir in the flour and cook for 1 min.
Add the sugar and ground black pepper and stock cube, then gradually stir in 425ml water. Bring to the boil, and simmer for 2mins. Whilst the gravy simmers, put the peas in a boiling pan of water and cook for 5 minutes.
Drain the potatoes, return to the pan, then add the butter, milk and mash until smooth and lump free. Season to taste.
Divide the potatoes between four plates, top each with 2 sausages and a little gravy, then hand the remaining gravy around in a sauce boat. Serve with peas.