Meanwhile, place the Brussels sprouts in the feed tube of a food processor fitted with the large slicing disk. (You don’t need to core them.) Process the sprouts in batches until they are all sliced, transferring them to a bowl. Add the Brussels sprouts to the pan with the shallots, add 1 1/2 teaspoons salt and 3/4 teaspoon pepper and saute for 5 to 7 minutes, stirring occasionally (I use tongs), until crisp-tender and bright green. Add the pancetta and cook for 1 more minute. Off the heat, stir in the balsamic vinegar, season to taste, and serve hot.