Steam the potatoes in a steamer basket set over boiling water for 15 to 17 minutes or until just tender. Let cool in a single layer on a towel on the counter. Gently press, using the heel of your palm, to crack the potatoes and flatten slightly on both sides.
Place the oil in a large (14-inch) cast-iron or other heavy skillet over medium heat. Add the potatoes and start to crisp them. Add the onions and rosemary. Cook until the potatoes are crisp on one side, flip and cook until they are crisp on the other, shifting the potatoes and allowing the onions to caramelize as the potatoes cook.
Place the contents of the pan onto a small platter, season with salt and pepper, and serve.