Preheat the oven to 180°C/350°F/Gas 4.
Put the oil or fat into the roasting dish, it should be about 1 cm deep. Slice the potatoes into 2 cm thick rounds. For the moment ignore the smaller rounded end pieces, and arrange the larger central slices tightly in the roasting dish; fill the gaps with the ends. Season and bake in the oven for 30 minutes. Very carefully remove from the oven and lift one potato from the centre to see if it is golden brown. If so, turn all the potatoes and return to the oven until tender, probably about 15 minutes. By this time most of the oil or fat will be absorbed and hopefully not too many of the potatoes will have stuck. Chisel them out carefully and arrange on a hot serving plate. Sprinkle with a little vinegar and sea salt.