Preheat the oven to 350°F. Thinly slice the potatoes and onion with a mandolin or sharp knife. Layer the potato and onion alternately in a 8x4 inch deep dish, sprinkling parsley, salt and black pepper between each layer. Finish with a layer of potato.
In a saucepan, heat the chicken stock and pour it over the top of the potatoes. Dot with butter. Cover the dish with foil and bake on the middle rack of the oven for 30 minutes, then remove the foil and press down on the potatoes to keep them submerged in the stock. Bake for an additional 30 minutes, or until the potatoes are tender and the top is golden brown. Remove from the oven and serve piping hot.
*REMEMBER... if you're using store bought chicken stock, unless it's low sodium, it can be pretty salty so add salt sparingly between the potato layers.