Dry fry the sausages in a frying pan for 15 – 20 minutes turning frequently. Add a little oil if they stick to begin with but you should not need more than a little drizzle.
When the sausages are cooked transfer to a warm oven 150℃ /130℃ /gas mark 2 to keep warm.
When the sausages are cooked add the onions to the pan in which they were cooked. The sausages are should have produced enough fat to cook the onions but if you are using low fat sausages add a little oil. Fry the onions over a low heat for 10 minutes until very soft.
Stir in the sugar and flour. Gradually add the wine and cook for a minute or two, then stir in the stock.
Cook stirring until the gravy thickens.
Meanwhile cut the potatoes into even sized chunks. Place in a saucepan with just enough water to cover. Add a pinch of salt and bring to the boil. Reduce the heat, cover and cook for 10–15 minutes or until just tender.
Drain well and mash. Heat the butter and milk together in a separate pan add to the potatoes, and mash again to mix well.
Serve the sausages and mash with the onion gravy spooned over.