Cut onions in half vertically, then cut halves into 1/8th inch slices. Place in a large skillet in one inch of water. Bring to a boil, cover and cook over high heat for 5 minutes. Uncover, reduce to medium heat, and cook another 5 minutes. Drain off water. Over low heat, stir in snipped fresh rosemary leaves and enough butter to coat. Toss and serve immediately.