Unlike many potato gratins, which are rich with cream and cheese, this light one is flavored with stock.
Preheat the oven to 375°. Melt the butter in a medium saucepan. Add the onion and thyme, season with salt and pepper and cook until softened.
Generously butter a 9-by-13-inch glass baking dish. Spread half of the potatoes in the baking dish and season well with salt and pepper. Spread the onions on top and cover with the remaining potatoes. Season well with salt and pepper. Pour the hot stock over the potatoes and press gently to even them out. Bake in the top third of the oven for 1 hour, or until tender throughout and crisp on top.