1. Preheat the oven to 180°C/350°F/gas 4.
2. Wash and trim the Brussels sprouts, clicking off any tatty outer leaves.
3. Cook for 8 minutes in a pan of boiling salted water, then drain well.
4. Squeeze the chorizo out of its skin, crumbling it into a roasting tray with 1 tablespoon of olive oil.
5. On the hob, fry over a medium heat for 6 minutes, until you have ruby oil, then crumble in the vac-packed chestnuts, strip in the rosemary leaves and shake about.
6. Tip in the sprouts and toss in the sherry vinegar. Squash and flatten the sprouts with a potato masher so they suck up more flavour.
7. Roast for 25 minutes, or until starting to colour. Season to perfection, bang out and serve.
