Heat the oven to 190C (170C fan). Rub a medium baking dish with oil and set aside.
Layer half the sliced potatoes over the base of the dish, overlapping the slices slightly like roof tiles. Scatter over half the onion and half the corned beef, then season with salt and pepper.
Repeat the layers with the remaining potatoes, onion and corned beef, finishing with a tidy layer of potatoes on top.
Pour the hot stock slowly over the layers so it seeps down through the dish. Cover tightly with foil and bake for 45 minutes until the potatoes are tender when pierced with a knife.
Remove the foil, scatter over the grated cheese and bake for another 15 to 20 minutes until the top is golden and bubbling.
Let the dish rest for 5 minutes out of the oven before slicing into wedges and serving.