Preheat oven to 350=B0. Heat butter in 25 cm skillet until melted. Stir in onions, rosemary, salt and pepper. Cook over low heat, stirring frequently, until onion is very soft and begins to brown, about 20 minutes. Arrange half of the potato slices in greased 10 x 4 cm pie plate. Top with half the onion mixture. Repeat layers. Pour half and half over top. Cover with foil. Bake for 1 hour. Uncover and cook until top is golden brown, about 20 minutes longer. Cut into wedges to serve.