Step 1Put the cipollini in a large bowl and pour over enough boiling water to cover. Let them stand for 2 minutes (if using pearl onions, 1 minute should be enough). Drain the onions and run them under just enough cold water so they're cool enough to handle. Cut off the stem end of each and peel back the papery skin, which will now be quite soft. In some cases, the top layer will peel off as well. Trim the dark, dry part of the root end, but do not trim too deeply or the cipollini won't hold together as well.
Step 2Dice the pancetta and place it in a skillet with the olive oil over medium-low heat. Cook until the pancetta has rendered its fat and browned, about 15 minutes.
Step 3Pour off all but 1 to 2 tablespoons of the fat and add the peeled cipollini to the skillet. Toss to coat with the fat and add the balsamic vinegar, one-fourth cup chicken broth, the rosemary and one-half teaspoon salt. Cover and cook over medium-low heat until the cipollini are tender enough to be easily pierced with a knife, about 20 to 30 minutes (about 20 for pearl onions).
Step 4Remove the lid and increase the heat to high. Cook, stirring the cipollini to keep them from scorching, until they have colored slightly and a dark brown crust has formed on the bottom of the pan. Do not let this burn. Add another 2 to 3 tablespoons of chicken broth and stir, scraping the bottom of the pan to release the browned bits and glaze the cipollini. Season with more salt, a splash of balsamic to taste and a generous grinding of black pepper.