Heat oil and butter over medium heat in a large saute pan until it is hot and just starting to brown. Add the potatoes, let them sit for a moment to brown and then every minute or so give the pan a shake turning the balls over. This will let them brown evenly. After about 10 min. into cooking add the rosemary and continue to cook until the potatoes are tender inside, about another 5-10 min. Remove from heat and season with salt and pepper. Serve immediately.