Place the potatoes into a pot of cold lightly salted water, with plenty to cover. Bring to the boil. Add the sliced onion and cauliflower, and cook at a fast simmer for 8 to 10 minutes, just until the potatoes are tender. Drain carefully and set aside for about 10 minutes.
Preheat the oven to 425°F.
Butter a shallow casserole dish. Layer half of the potatoes in the bottom, all of the onion and half of the cauliflower. Season with a good grinding of black pepper. Top with half of the cheese. Top with the remaining potatoes and cauliflower. Grind some more pepper over top. (Salt only if you think you will need it. I never do.) Pour the chicken stock over top, cover with the remaining cheese and dot with butter.
Bake in the heated oven for 20 to 25 minutes until golden brown and bubbling. Serve hot.
