Pre-heat the oven to 180C. Butter a large, shallow oven-proof dish lightly.

Scrub the potatoes well and slice into thin slices (5mm or less). Crush the garlic and slice the onion into thin rings. In a jug, mix the cream with the onion soup powder.

Lay a single layer of potato slices on the base of the buttered dish. Scatter a little crushed garlic, some onion rings and some sliced mushrooms over the top. Pour about about a quarter of the cream mix evenly over the vegetables, then sprinkle with freshly ground black pepper, a little thyme, and salt (but go easy on the salt as the soup is already salty).

Repeat until the ingredients are used up - you will probably make at least 4 layers. Cover the dish with aluminium foil and place in the pre-heated oven for an hour to 90 minutes, or until the potatoes are soft and most of the liquid has been absorbed.

Turn on the grill, remove the aluminium foil from the dish and spread the grated cheese on top of the final potato later. Sprinkle the breadcrumbs over the top and return to the oven, under the grill.

Grill until the cheese is melted and the crumbs are beginning to brown, then remove from the oven and serve.