Preheat the oven to 220C/425F/Gas 7.
Place one tablespoon of the oil into a frying pan over a medium heat and fry the onions until softened and beginning to turn golden-brown. Remove them from the pan and set aside.
Add the carrots and celery and fry until softened and starting to turn golden-brown. Remove and set aside with the onions.
Heat the remaining tablespoon of oil and 50g/2oz of the butter and allow the butter to foam. Add the minced mutton, stirring it until it browns.
Add the carrots and celery and stir well.
Add the beef stock and crumble in the stock cube. Stir well to combine.
Add the thyme and parsley and stir to combine.
Mash the potatoes with the remaining butter in a clean bowl. Season with salt and freshly ground black pepper and add the fried onions.
Transfer the meat into an ovenproof dish. Spread it out to make an even layer. Spoon over the mashed potato and tease the surface with a fork to create some peaks.
Place the dish into the oven and bake for 30 minutes, or until the topping is golden-brown and crisp. Serve at the table in the dish.