1. Put the potato segments in a pan of salted water and bring to the boil. Simmer for 10-15 minutes, or until you can put a knife through the potato.
2. Heat up the grill and put the sausages under it. Turn them regularly until they are an even golden brown colour all over. This should take about 15 minutes.
3. Once the potatoes are cooked, drain and return to the pan. Add the butter and mash until all the butter has melted, then stir in the milk. Mash until almost smooth (if you want a completely smooth mash then put through a sieve) then stir in the mustard, spring onion and parsley. Season to taste with salt and pepper.
4. To make the gravy, melt the butter in a pan and once it starts to bubble, add the onion and a sprinkling of salt. Fry for 5 minutes until soft, then stir in the flour and cook out for a minute, being careful not to let it stick and burn. Gradually pour in the stock and stir until you have a thick gravy.
5. Arrange the sausages on top of a mound of mash, then top with the onion gravy.