Take the time to slice the potatoes and onions very thinly. This will help create really delicate layers, which will taste delicious.Preheat the oven to 350°F. Butter or oil ovenproof dish around 12 in x 8 in and about 2 in deep.Arrange a layer of sliced potatoes over the base, then a layer of sliced onions*, followed by a scattering of sliced sage or thyme and a sprinkle of salt and pepper. Continue in this way, alternating the layers of potatoes and onions, finishing with a layer of potatoes that slightly overlap each other.Pour your stock over the potatoes. Season the top layer with a little salt and pepper, then scatter over the whole sage leaves and your Parmesan, if using. Pop little dots of the butter all over the potatoes or drizzle with olive oil, place the dish on the highest shelf of the oven and cook for around an hour, until the top is crisp and golden and the potatoes cooked all the way through.*For an elevated dish the onions may be sauteed in olive oil or butter before the layering process to bring out their sweetness. When slightly carmelized around the edges then layer as above with potatoes. Use stock to loosen brown bits from sautee pan before pourong the stock over potatoes.Literally called "potatoes from the baker," pommes boulangère originated in France centuries ago when people in rural areas did not own ovens of their own. On their way to church, women would take the Sunday roast of lamb in a dish, surrounded with sliced onions and potatoes, to the baker to be cooked in his oven while the family prayed. Afterward, they picked it up and took it home for lunch, perfectly done.