Preheat oven to 350 degrees. Slice potatoes approximately 1 cm thick. Select a casserole that is wider than it is high, so that most of the slices can be layered across the bottom. Butter the bottom of the casserole and place the slices in the pan. Slice the onion thinly and scatter under and over the potatoes. Add salt and pepper; dot generously with chunks of butter. Seal the pan tightly with aluminum foil and bake for 1 to 1.5 hours, or until the potatoes are tender and slightly browned. Remove foil to serve and top with a good sized dollop of sour cream, sprinkled with a pinch of herbs. Pass separately a bowl of sour cream with herbs. Serves 8.