Peel potatoes and cut in half lengthwise (you only need enough that will fit in the bottom of your baking dish- so start with four and see if you need to peel more to fill the dish).
Place butter/margarine in a 9x13 baking dish and place in a preheated 350 F degree oven, and let it melt. Remove from oven and add potatoes in coating thoroughly (you may have to use a basting brush for this). Arrange potatoes cut side down, and season with the salt and pepper.
Return dish to oven and bake for 45 minutes to an hour (depending on thickness of potatoes), or until the potatoes are golden brown and cooked through. For best results baste the potatoes with the melted butter in the bottom of the dish halfway through the cooking time.
Note: this is a really easy side dish if you are cooking other items in the oven and can easily be made when you have to bake something between 325 and 375- just adjust the cooking time.