Peel and thickly slice the potatoes into about 1.5cm slices. Boil them in salted water for 6-8 minutes until just tender, then drain.
While the potatoes are boiling, heat the oil in a large frying pan. Fry the onion over a medium heat for 5 minutes until it becomes transparent. Add the bacon to the pan and carry on frying for 5-7 minutes, stirring occasionally, until the onion turns lightly golden. With a slotted spoon, remove the onions and bacon from the pan to a bowl, leaving as much fat in the pan as possible.
Heat the grill to high. Add the potatoes to the pan and brown briefly in the fat. If your potatoes are on the floury side they may break up or stick a bit, but this isn’t a problem, just keep everything moving and avoid too many burnt bits. Return the onions and bacon to the pan and lightly mix everything together.
Nestle the chunks of cheese among the potatoes and bacon then drizzle with the cream. Sprinkle with the chilli, if using, then evenly scatter over the breadcrumbs. Protect the pan handle with foil if necessary and grill for 5 minutes, until lightly browned and just on the point of bubbling.
Transfer to plates with a wide spatula – no hurry, the dish will not spoil – and savour with a glass of wine.