Cook potato in boiling, salted water for 15 minutes, until soft. Drain and coarsely mash with butter and hot milk. Season.
Meanwhile, cook onion in a medium saucepan on low heat with 2 tablespoons water, stirring frequently for 8-10 minutes, until browned and very soft. Stir in stock, reduce heat to low and cook for 4 minutes, until sauce thickens slightly.
Meanwhile, heat a non-stick frying pan or grill on medium and cook sausages for 10 minutes, turning, until browned and crisp on all sides. Serve sausages with mash and onion gravy.
great old stand by one