First place the roasting tray with the fat in it on the highest shelf of the oven while it pre-heats. Thinly peel the potatoes using a potato peeler, then leave the small ones whole and cut the larger ones in half.
Put them in a steamer fitted over a large pan of boiling water, sprinkle the salt all over them, put a lid on, turn the heat down to low and steam the potatoes for about 10 minutes. After that lift one out with a skewer and see if the outer edge is fluffy.
You can test this by running the point of the skewer along the surface – if it stays smooth, give it a few more minutes.
Then drain off the water (reserving some for the gravy if you like).
Place the lid back on the saucepan, and, holding the lid on firmly with your hand protected by a cloth or oven glove, shake the saucepan vigorously from side to side then turn the pan upside down and give it another shake.
This shaking roughens up the cooked edges of the potato and makes them floury and fluffy – this is the secret of the crunchy edges.
Now, still using the oven glove to protect your hands, remove the hot roasting tray containing its sizzling fat and transfer to the direct heat (medium) on the hob. Then use a long-handled spoon and quickly lower the potatoes into the hot fat.
When they are all in, turn each one over a couple of times so it's completely coated with fat.
Now place them back on the highest shelf of the oven and leave them unattended for 50 minutes to 1 hour, or until they are golden brown.
There's no need to turn them over at half-time – they will brown evenly by themselves.
Sprinkle them with a little crushed salt before serving straight away; they lose their crunch if you keep them waiting. If they're ready before you are, turn the oven off and leave them inside.