Crisp Roast Potatoes

Heat the oven to 220C/425F with a large heavy roasting tin on the middle shelf.

Bring the 2 litres of water to a boil in a large pan with the 20g of fine salt and the bicarbonate of soda.

Use no more bicarbonate of soda than the teaspoon. It turns the water alkaline, which breaks the pectin at the surface of each piece down into the starchy slurry that later crisps; twice as much takes the potatoes past that and they collapse in the pan and taste faintly soapy.

Add the potato pieces and bring the water back to a boil.

Simmer for 8-10 minutes, until a knife tip goes 5mm into a piece easily and meets firm resistance at the centre.

Drain them in a colander.

Leave them to stand for 2-3 minutes, until the steam has stopped rising and the surfaces have gone matt and chalky.

Tip the pieces back into the empty dry pan.

Put the lid on and shake the pan hard three or four times, until the outsides are ragged and coated in a paste of their own starch.

Stop shaking there. Those ragged edges are the entire surface you are about to crisp, but keep going and the pieces break into rubble that browns to nothing.

Take the hot tin out of the oven and add the dripping to it.

Return the tin to the oven for 5 minutes, until the fat is thin, loose and shimmering.

Lower the potatoes into the hot fat one piece at a time, standing back from the tin as they go in.

Turn each piece over so every face is coated in fat.

Space them out with a couple of centimetres of bare metal between them. A packed tin traps the steam coming off the potatoes and they come out pale and soft, so use two tins rather than crowding one.

Roast for 25 minutes without touching them.

Turn each piece onto a fresh face with a spatula, scraping up any that have stuck.

Tuck in the thyme sprigs and the crushed garlic cloves.

Roast for a further 20-30 minutes, until the potatoes are deep brown all over and rasp audibly when pushed with a spoon.

Lift them out of the fat onto a warm serving dish with a slotted spoon.

Never cover them or pile them into a deep bowl. The crust is thin, and steam from the potatoes underneath softens it within minutes.

Scatter with flaky salt and serve at once, alongside the roast chicken with lemon and thyme or anything else with gravy.