Brown the mince in a large frying pan over a medium-high heat, breaking it up with a wooden spoon until no pink remains, then tip away the excess fat.
Add the onion and carrots and cook for 5 minutes until the onion has softened. Stir in the flour, the crumbled stock cube and the tomato puree, then pour in the hot water and the Worcestershire sauce. Simmer for 15 minutes until the carrots are tender and the gravy has thickened. Stir in the peas for the last 2 minutes, season well and tip into an ovenproof dish.
Meanwhile, boil the potatoes for 15 to 18 minutes until tender, drain well and leave them to steam dry in the colander for a couple of minutes. Mash with the butter and milk and season generously.
Spoon the mash over the mince and rough up the top with a fork, raking deep ridges all over so they crisp up in the oven.
Bake at 200C (180C fan) for 20 to 25 minutes until the ridges are golden and the gravy bubbles at the edges. Rest for 5 minutes before serving.