Mix together melted butter and oil, then pour into a 9x13 inch baking dish. Place the potatoes, onion and garlic into the dish, and stir until coated. Sprinkle with rosemary, salt, and pepper. Cover with aluminum foil.
Bake in the preheated oven for 30 minutes, or until the potatoes are tender. Stir the potatoes occasionally to ensure even cooking.