Put the potatoes into a large saucepan of cold water, bring to the boil and simmer for 15 minutes until tender.
Meanwhile, fry or grill the sausages over a medium heat for 10 minutes, turning to get an even colour. Add the onion and apple wedges and cook with the sausages for 6–7 minutes.
Drain the potatoes well. When they are cool enough to touch, peel them, then mash well so they are nice and creamy.
Add the butter, mustard, garlic and a good sprinkling of salt and pepper to the potatoes, and carry on mashing. Taste and add more mustard if you want. Finally, stir in the parsley and olive oil.
Transfer the sausages, onion and apple to a serving plate. Pour off the excess fat from the pan to leave about 1 tablespoon, then mix in the flour. Gradually stir in the stock, bring to the boil and stir until thickened. Season and strain into a jug.
Pile the mash up on a plate and stick the sausages and onion wedges on top. Spoon over the gravy and serve.