Heat oven to 160C/fan 140C/gas 3. Slice the potatoes into rounds the thickness of a £1 coin. Heat the oil in a large saucepan and add the onions, season with a little salt, lots of pepper and the thyme. Cook gently for about 15 mins until softened.
Generously butter a large roasting tin and arrange a layer of potatoes over the base, overlapping each slice with a sprinkling of onions. Drizzle with a little of the melted butter and season. Continue layering, drizzling with butter, sprinkling with onions and seasoning until all the potatoes have been used, finishing with a top layer of potatoes. Cook in the top of the oven for 1 hr – the potatoes should be meltingly soft and the top golden and crusty.