Pre-heat the oven to 180 Degrees C Fan.
Peel the potatoes, cut them into small chunks, and cover them with cold water in a saucepan. Bring the saucepan to the boil, and then simmer the potatoes until soft about 10 – 15 mins.
While potatoes are cooking, gently heat the olive oil in a large pan, add the onion and sauté until softened for about 3 – 4 minutes.
Add the beef mince and gently cook the mince until no longer pink breaking it down with a wooden spatula for about 5 minutes. Season to taste.
Stir in the stock, frozen vegetables, tomato puree, Worcestershire sauce and simmer for 5 minutes.
Cover the bottom of a shallow ovenproof baking dish with the beef and veg mixture.
Drain the cooked potatoes, tip them back into their saucepan and mash together with the butter and milk. Season to taste.
Spoon mash over the beef mince mixture in the dish. Smooth down with back of a spoon. Then create patterns with a fork.
Bake for about 20 – 25 mins until lightly browned.
Allow to rest for 5 – 10 minutes before serving.