Slice the potatoes lengthways into rounds the thickness of pound coins.
Heat the oil and butter in a shallow 22cm saute pan over a low to medium heat. Add the onion and gently fry until golden and soft.
Add the potatoes together with the garlic, thyme a little sea salt and some pepper. Toss gently to coat.
Reduce heat to as low as possible and cover the pan so that the potatoes cook slowly, stirring now and then. After 40-50 minutes they will be soft and golden.
Slice the cheese thinly, lay it on top of the potatoes and cover the pan again. After a couple of minutes the cheese will be melted . Serve immediately.