Preheat the oven to 200ºC, fan 180ºC, gas mark 6.
Heat the oil in a frying pan and brown the mince. Add 400ml water, then crumble in the beef stock cube, add the thyme and simmer for 10 minutes, stirring occasionally.
Add the chopped carrots and onion, and continue cooking for a further 15 minutes, or until the mince is tender. Stir in the tomato ketchup and Worcestershire sauce, then simmer for 10 minutes, stirring occasionally.
Meanwhile, cook the potatoes in boiling water until soft (10-15 minutes). Drain and mash with the milk, mustard and grated cheese, reserving a third of the cheese. Season to taste.
Add the cornflour to a little water and mix into the mince to thicken slightly, then season with salt and freshly ground black pepper.
Spoon the mince and vegetables into an ovenproof dish. Top with the mashed potato and sprinkle with the remaining cheese. Cook for 15-20 minutes or until the potato is golden.
Serve with peas for a hearty dinner.