Preheat the oven to 170C/325F/gas 3.
Add the oil to the casserole and bring to a fairly high heat. Add the onion, carrot and celery, reduce the heat to moderate, cover and gently sweat until the onion is translucent and the carrot is becoming tender. Remove from the heat.
Add the dripping or lard to the large frying pan and place over high heat on the hob. Once the fat is sizzling hot add a quarter of the minced beef. This needs to be done in batches as an overfilled frying pan will result in the meat being steamed rather than fried.
Cook the meat until all the liquid has evaporated and the meat is a deep brown. This can take a while but the more the meat is caramelised the better the gravy! Place in the casserole. Continue with the remaining 3 batches, placing each batch in the casserole.
Stir the meat into the vegetables and add the beef stock, tomato puree and Worcester sauce. Season and bring to a vigorous simmer.
Remove from the heat, cover and place in the oven for two hours.
Take the casserole out of the oven and if the mince mixture is still quite liquid place on the hob on a high heat and cook until most of the liquid has evaporated off. Remove from the heat and allow to cool.
To make the potato topping, cook the potatoes in simmering, salted water for 15 to 20 minutes until tender.
Drain and place a potato or 2 at a time into the potato ricer and squeeze into the saucepan.
Add the butter, milk, egg yolks and seasonings and stir thoroughly into the potato to create a smooth, creamy mashed potato. (If you don’t have a potato ricer, drain the potatoes, return to the saucepan and hand-mash. Once the potatoes have all been broken down add the butter, milk, eggs and seasoning and mash until smooth.)
Place the minced meat mixture into the shallow baking dish and spoon the potato over the top, starting at the edges before making your way into the centre. Smooth over with a palette knife.
Pull a fork over the mashed potato to make indentations, first vertically and then diagonally or horizontally (whatever you wish) and add a few small knobs of butter over it.
Place in the oven and bake for 30 to 45 minutes until the mash is golden and the filling bubbling hot.
