Add the potatoes to a saucepan of boiling water and simmer until soft but not crumbling.
Brown the sausages however you prefer; grill or fry. Put to one side. Once cooled slice into small bite-sized pieces. Fry mushrooms until softened.
In a large saucepan add oil then onions and fry until clear and soft. Add the curry powder and stir for a further minute. Add the potatoes and stir until coated in the paste. Add the sausages and mushrooms then stir again.
Cover the ingredients with boiling water, tomato sauce and tomato paste then stir. Simmer for 10 minutes. The potatoes should have just started to break down to create a nice thick sauce.
Leave for at least 30 minutes then serve the curry with basmati rice.