350 deg F

Add oil, butter, rosemary, vinegar, salt and pepper in a roasting pan.
Boil potatoes in salted water until just starting to break and flake.
Drain into colander..rough up the potatoes slightly by tossing.
Add to the pan and stir to coat all.
Roast for 40 mins, then bring out and lightly crush with a wire potato masher to pop the potatoes.
Back in oven for 10 to 20 mins