Place large baking tray in oven and preheat to 375ยบ F.
Parboil the cubed potatoes in salted water for 5 minutes. Drain, and return them to the warm pan to let them completely dry out of water. Dry potatoes will be more crispy.
Meanwhile, in a large frying pan, over moderate heat, melt butter and oil. Then and stir in parsley and garlic. Add cubed potatoes and toss together, cooking for 5-10 minutes, stirring occasionally to prevent potatoes from browning or sticking to the pan.
Remove rosemary leaves from their stalks and finely chop. Add them to the potatoes and season with salt and pepper.
Transfer potatoes to a baking tray in one layer and bake until they are golden brown and crispy on the edges. Turn them once after 20 minutes, and continue baking for another 15 minutes.
