Cut potatoes into 1/2-inch-thick slices.
Cook butter, oil, and rosemary in a small saucepan over medium heat, stirring until butter melts.
Brush 1/4 cup butter mixture over 2 baking sheets. Arrange potato slices evenly in a single layer on prepared sheets; brush with remaining butter mixture. Sprinkle with salt and pepper.
Bake at 450° for 35 minutes or until golden brown.