Preheat the oven to 200ºC, 190ºC fan assisted, Gas mark 6.
Brown the mince and onion in a large pan. Drain off any excess fat.
Blend the contents of the sachet with the water and stir into the mince, add the carrots and peas. Bring to the boil stirring continuously, then transfer to an ovenproof casserole dish.
Spoon the mashed potato over the mince and bake for 30 minutes.