Put the potatoes in a large pan of salted water, bring to the boil and cook for 15 minutes until tender.
Meanwhile, heat 1 tablespoon of oil in a large frying pan over a medium heat. Add the sausages and cook for 15-20 minutes, turning regularly, until browned and cooked through.
In a separate pan, heat the remaining oil and cook the sliced onion and diced apple for 5 minutes until softened and lightly caramelised.
Sprinkle the flour over the onion and apple, stir well and cook for 1 minute. Crumble in the stock cube, add the hot water and stir until smooth. Simmer for 3 minutes until slightly thickened.
Drain the potatoes and mash with the butter, milk, mustard, salt and pepper until smooth and creamy.
Serve the sausages on top of the mustard mash with the apple and onion gravy poured over.