Pre-heat oven to 200C.
Boil the potatoes in plenty of simmering, salted water. When they are just cooked, remove from the heat and cool in cold water. Drain, dice and leave to one side.
In a frying pan, fry the onions and thyme in the butter for about 3 minutes. Add the marmite, stock and red wine and reduce by half. Add the corned beef, parsley and some salt and pepper and cook for 5-10 minutes, breaking it up with a fork.
Season the cooled potatoes and mash them thoroughly with some milk and butter. Place the corned beef mixture into an ovenproof dish and top with the mashed potatoes. Grate the cheese and mix together with the breadcrumbs. Sprinkle the mixture over the mashed potatoes and bake for 20 minutes to cook through and brown the top.
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