Rub the brisket with 1 tbsp oil; season. Heat a large, heavy, lidded saucepan over a high heat and sear the brisket on all sides for 6 mins or until browned all over. Remove and set aside.
Reduce the heat to medium and toast the cumin seeds in the pan for 1 min until fragrant. Add 30g of the butter, the remaining oil and the onion, carrot and celery. Cook for 8 mins, stirring occasionally, until the mixture is softened. Add the garlic and cook for 1 min.
Increase the heat to medium- high, stir in the tomato pureee and cook for 2 mins. Add the wine and bubble for 2 mins.
Stir in the tomatoes and stock and then Place the brisket into the sauce. Bring to the boil, then reduce the heat to low, cover and cook for 3½ hrs, turning the meat every hour.
About 10 mins before the brisket is ready, boil the potatoes for 12 mins until soft. Drain and leave to steam for 2 mins. Mash until smooth and beat in the milk and remaining 50g butter and season.
Put the brisket in a roasting dish and use 2 forks to shred it. Set aside. Add the oregano to the sauce and cook over a high heat for 6 mins. Return the meat to the pan.
Divide the mash between the bowls and top with the ragu.