Thinly slice the potatoes and place in a greased shallow 2-quart baking dish. In a small saucepan, heat butter, onion, salt, thyme, and pepper until the butter is melted. Drizzle over potatoes. Cover and bake at 425° for 45 minutes or until tender. Sprinkle with cheese and parsley. Bake, uncovered, 15 minutes longer or until cheese melts.