Heat oven to 180C/160C fan/gas 4. Season the steaks well on all sides, then heat a frying pan with 2 tsp of the oil. Brown the steaks well on all sides, then transfer to a snug casserole or narrow, deep baking dish.
Tip the Guinness into the frying pan and bring to a simmer, scraping up any stuck meaty bits. Pour over the steaks along with the stock cube, Worcestershire sauce, brown sauce, bay and skin-on halved shallots (leave the skins on to save time – they are removed from the sauce later). Cover the dish tightly with foil first, then add the lid (or a flat baking tray). Bake in the oven for 3 hrs until the meat is meltingly tender. Can be done up to 1 day in advance, then cooled and chilled.
For the chips, 1 hr before serving, turn up oven to 200C/180C fan/gas 6. Bring a large pan of salted water to the boil while you cut the potatoes into as many 3cm fat chips (save the trimmings in a bowl of water and use for mash another night). Par-boil for 3 mins, then drain and let steam-dry for a few mins. Tip onto a baking tray, toss with the polenta or flour and some sea salt, then drizzle over the oil and toss again. Roast for 45 mins until really crisp and golden.
When the chips have 20 mins left, lift your steaks from their cooking liquid and use a few sheets of baking parchment to wrap them in a parcel, followed by foil. Put on a low shelf in the oven to heat through for 20 mins. Meanwhile, strain the cooking liquid into a pan, discarding the herbs and shallots. Boil fiercely until reduced by three-quarters. Taste for seasoning, then keep warm.
Whisk the chopped shallot, horseradish, remaining 2 tsp oil, vinegar and 1 tsp water with some seasoning, then use to dress the watercress.
To serve, carefully open the meat parcel over your saucepan to catch any juices, and add your beef to the sauce to coat. Stack your chips between 2 plates and add a pile of dressed watercress. Lift a steak onto each plate and spoon over the syrupy juices. Any extra can be served in a jug on the side.