The critical factor in potatoes, he says, is water content. A lot of his research for roasties overlaps with his quest for the perfect chip.

“I want the centre to be moist and fluffy and the outside to have that glass-like crunch,” he says. “If your water content is too high, your potatoes aren’t going to crisp. If your dry content is too high, the potatoes get crumbly and fall apart. Some potatoes are not fit for purpose. Things like this make people think they can’t cook.” (In his restaurants, Blumenthal currently uses Agria potatoes though he recommends Albert Bartlett Rooster, Burgundy Red or Maris Pipers as good all-rounders.)

He experimented with different methods of drawing excess water out of the potatoes – pin-pricking them, vacuum cooking them – only to conclude that the crucial element is the boiling. “That first cooking stage in the water is crucial,” he says. You want to peel them, cut them into quarters or eighths to maximise corners, and rinse them, before an intense boil.

“People say ‘parboil’, but you want to cook them quite heavily until they start to break up, skimming off any excess starch that builds up. It might sound counterintuitive to dry potatoes out by simmering them in boiling water, but with all the cracks and fissures there’s a much bigger surface area for crispness, and the oil and fat can run into them.”

Rather than tipping them out into a colander to steam-dry, he recommends lifting them carefully with a slotted spoon onto a drying rack, so they have room to cool. No chuffing is required; if you have boiled them correctly, the violent shaking would destroy them.

“As they cool, the starch crystallises on the outside of the potatoes, so they become harder and sturdier again.” This is the moment where you could keep them in the fridge or the freezer and roast them the next day.

He recommends cooking them in olive oil which has been pre-heated in an oven to 180C. “You have to use enough oil,” he says. “You’ve made all the cracks, so if you haven’t got enough oil they won’t go crispy. I use about a centimetre of oil in the tray.

Then cook at 180C for between an hour to an hour and 20 minutes, turning occasionally. Right at the end, add rosemary and garlic, just to release the flavours.”

Put them back in the oven for 15 to 20 minutes and there you have it. And the best thing about perfect roast potatoes is they are not just good for Christmas, they are a skill for life, too.

The Sunday Roast ‘Potato Time’ is available on Sundays at Dinner by Heston Blumenthal in London from October 13, priced £98 ($210) per person