First, start by making the gravy. Melt the oil and butter in a large saucepan over a gentle heat. Add the onion and cover with a lid. Cook slowly for approximately 10 minutes or until the onions are soft.
Add the sugar and balsamic vinegar to the onions and stir well. Cover with the lid and continue to cook for a further 5 minutes.
Add the stock and boil gently uncovered for 5 minutes.
In a heatproof jug or bowl mix the cornstarch/flour with the cold water to a thin paste.
Pour a little of the hot gravy into the starch mixture and mix thoroughly. Pour the starch mixture back into the gravy, raise the heat to high and boil for 10 minutes or until the gravy is slightly thickened. Keep warm until ready to serve.
Meanwhile, start the mashed potato by boiling the potatoes in lightly salted water until soft.
Drain, and keep warm until ready to mash.
While the potatoes are cooking, cook the sausages. Heat the oil in a large frying pan, turn the heat to medium and add the sausages. Fry until the sausages are golden brown and firm, turning them from time to time, about 20 minutes.
Once cooked, place in an ovenproof dish and keep warm until the mash and gravy are ready.
Finish the mash by placing the milk and butter in the pan used to boil the potatoes, return to the heat and warm gently until the butter has melted.
Add the potatoes and mash using either a potato masher, a fork, or a potato ricer.
Whip the mashed potato lightly with a wooden spoon. Season with salt and pepper.
Spoon the mash onto four warmed dinner plates, place two fat sausages on top of the mash, and pour the gravy over the dish.
Serve and enjoy!