Boil potatoes for 15 – 20 minutes in boiling water and then drain immediately (or they will go soggy).
Meanwhile fry the onion gently in 1 tbsp of oil in a large pan for 5 minutes.
Add the mushrooms and fry for another minute.
Add the tomatoes, ketchup, mustard powder, herbs, stock and pepper.
Bring to the boil then simmer for 5 minutes.
Transfer the mixture to a dish.
Mash the potatoes with the butter, salt and pepper.
Place the mash on the meat, (I find piping is the easiest option), then fluff up with a fork if desired.
Dot the mash with flecks of butter and bake in the oven at 200°C for 25 minutes or until browned.
Serve with your favourite vegetables.