Microwave the potatoes for the time instructed on the package, minus 2 minutes. Allow to cool slightly to handle, then cut the longer potatoes in half, leaving the smaller potatoes intact.
In a large pan on medium-high heat, add the oil, butter and rosemary stalks. Using the bottom of a dry cup measure or something similar, gently smash each potato portion to about 1/4 inch thick and scrape off with a butter knife. Slide a smashed potato right into the bubbling fat and repeat with the remaining potatoes. Cook until the potatoes are golden and crisp on one side, about 3 minutes, then flip and continue to cook, adding more butter if needed, until golden on the other side, about another 3 minutes. While cooking, mind the rosemary stalks and flip and turn them as well. Once they are lightly crisped, remove them to a drying rack above a baking sheet along with any crisped potatoes. Season gently with salt and pepper.
Crinkle the rosemary leaves with your fingers or chop gently and add to a large metal bowl with the parsley. Coat the inside of the bowl with the garlic. Transfer the potatoes to the bowl and toss gently until coated. Serve warm.