Heat the oil over a medium to high heat in a large non-stick pan/wok and fry the beef and onions until beef starts to brown (10 - 15 minutes).
Drain the fat (a colander is the easiest way to do this).
Stir in the flour and cook for a minute.
Add the stock, horseradish sauce, Worcestershire Sauce, mixed vegetables and ground pepper (no need for salt as stock and cheese have plenty).
Bring the pan to the boil and then reduce to a simmer for about 15 minutes with a lid on removing it for the last 5 minutes until you have a meaty sauce.
Preheat the oven to 200°C.
Meanwhile microwave the mashed potato according to the instructions (about 9 minutes in total) and then beat in the butter, milk and half the grated Cheddar.
Put the meat sauce into a dish, spoon over the potato and then fluff up with a fork whilst filling in the gaps.
Sprinkle on the rest of the grated Cheddar and place in the oven for about 25 minutes until top golden and meat bubbling.
Serve with more mixed vegetables.