Peel potatoes and cut to preferred size (quarters work well). Put fat or oil into a heavy roasting pan, then into the oven where your joint is roasting, 190 degrees Celsius, until fat is very hot (at least 10 mins). Parboil potatoes for 5-7 mins in boiling water (salted if you wish) until potatoes are soft on the outside, but not cooked through. Drain well, reserving liquor for gravy later if you like. Return potatoes to empty saucepan, put lid on, shake vigorously until outside of potatoes are slightly broken up. This gives the extra yummy, super crunchy outside.
Bring roasting tin out of the oven, and put on hot stove top to keep oil piping hot (or work really fast!). Pop potatoes into roasting pan, and coat in hot fat.
Sprinkle with salt if you wish.
Return to oven and cook for 40 mins or so, turning at least once, until potatoes are golden and very crispy.