Make the pan quite hot. Fry the sausages in a tiny bit of oil- lots more oil will come from the sausages.
While the sausages cook cut the onions across the grain to make rings
Get the sausages nice and brown on the outside. It doesn't matter whether they are cooked inside as there is plenty of time for this later
Add the onions and keep frying
Cook until the onions are soft and transparent
Now add the pint of stock/water, 2 teaspoons of tomato purée, teaspoon of mustard and dash of Worcestershire sauce.
The sausages should be pretty much covered otherwise add more water. Simmer slowly for at least 1/2 an hour. An hour is better. If they get too dry and start sticking add more water. Stir occasionally
Half an hour before you want to eat put some potatoes in water. I am cooking for one here, so you will probably want more potatoes than this for two (I suggest 4)
Cover and bring to the boil. Turn the heat down and simmer for 20 minutes
Keep checking and occasionally stirring the sausages. There should be less sauce now.
Drain the potatoes and add a little butter and milk.
The sauce should be pretty thick now. If it isn't turn up the heat to reduce it. If it is too thick add some water. It is good to have enough liquid to soak into the potato.
Serve the mash topped with the sausages and enjoy.