Heat oil in a large frying pan and fry onion until softened. Add mince and stir until browned.
Add mixed vegetables to the pan. Mix contents of packet with 300 ml water and add to mince. Stir well until mixture boils, then simmer for 10 minutes, stirring frequently.
Transfer to an ovenproof casserole dish, top with mashed potato and cheese. Bake uncovered in a preheated oven at 180°C for 20 minutes, or until golden brown.