In a large pot of salted boiling water, blanch the onions for 1-2 minutes. Drain in a colander and rinse with cold water. When cool to the touch, peel the onions by trimming away the root end from each onion and slipping off the skin.

Preheat the oven to 350°F. Put the potatoes and onions in a baking dish large enough to hold them in a single layer. In a small bowl, combine the vinegar and butter and pour over the potatoes. Sprinkle generously with salt and pepper and toss to coat evenly. Nestle the 2 rosemary sprigs among the potatoes and cover with a lid or aluminum foil. Bake, turning every 20 minutes, until the potatoes are tender, about 1 hour. Remove aluminum foil for the last 20 minutes.

To serve, divide among 6 small bowls or plates, garnish each with a rosemary sprig.