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.