Put each potato in the bowl of a wooden spoon, like you would carry an egg in an egg-spoon race, and cut across at about 1/2-inch intervals.
Put cut potatoes on the baking pan on the stove with the butter and oil and heat till sizzling.
Turn potatoes well, putting them in upside down (i.e., cut-side down) first, then right side up, and spoon the fat over them.
Sprinkle each potato well with salt and put in oven. Cook large potatoes for about an hour and 10 minutes, testing to see whether the flesh is soft. (Forty minutes for the new potatoes.)
Transfer to warmed plate and serve.