Preheat the oven to 375°F (190°C or 180°C for a fan oven).
Heat the oil over medium heat in a large frying pan.
Sauté the onion until it begins to soften a little.
If using raw vegetables, add them now and cook until they begin to soften a little.
Stir in the cooked, chopped beef or lamb.
Warm gently for a minute or two and then sprinkle with the flour. Stir.
Gradually add the stock, a bit at a time, stirring as it thickens. Go gently, you can always add more liquid. When you have a fairly thick gravy, stir in the Worcestershire sauce, tomato puree or ketchup.
If you are using leftover vegetables or frozen vegetables, add them now.
Check the thickness of the gravy, gradually adding a bit more stock if necessary.
Stir in the salt and pepper.
Transfer the mixture to a 9 x 13 inch casserole - or into four smaller individual casseroles.
Spoon the mashed potato over the top of the meat and vegetable mixture and then use a fork to spread it over the meat mixture, right up to the edges.
Place the casserole(s) on a baking sheet and bake in the oven for 20 to 30 minutes or until the mashed potato begins to turn golden brown.
Sprinkle with a little parsley or paprika to garnish before serving.