Scrub the potatoes well and pierce with a fork. Cook on full power in microwave oven for 5 minutes; turn the potatoes over and cook 5 more minutes. Remove the potatoes from the oven and set aside to cool. Thinly slice the cooled potatoes.
Cook and stir the butter and onion in a large frypan over medium heat until the onion is translucent, about 5 minutes. Gently place the potatoes into the frypan and fry, carefully flipping the potatoes a few times until slices are browned around the edges. Sprinkle with salt and pepper.