Heat the oil in a frying pan and sauté the onions for about 5 minutes until softened and beginning to colour. Transfer to a small casserole with a slotted spoon.
Add the bouquet garni or herbs and orange zest to the casserole and set aside.
Increase the heat and brown the brisket on all sides in the frying pan, then place on top of the onions.
Deglaze the pan by adding the orange juice to the pan and stirring well. Add the beef stock and bring to the boil.
Pour the hot stock over the brisket and cover. Cook in the oven for 2–3 hours.
To serve, lift the beef out of the casserole. Place on a board and serve cut into thick slices. Remove the bouqet garni and discard. Season to taste and serve the onion gravy spooned over the sliced meat.