BUTTERNUT SQUASH WITH SHALLOTS AND SAGE

2 tablespoons olive oil
3 shallots, halved lengthwise, then cut crosswise into 1/4-inch-thick slices (3/4 cup)
1 (1 3/4-lb) butternut squash, peeled, halved lengthwise, seeded, and cut into 1/2-inch cubes (4 cups)
1/2 cup chicken broth or water
1 tablespoon packed brown sugar
1/2 teaspoon finely chopped fresh sage
1/2 teaspoon salt
1 teaspoon balsamic vinegar
1/4 teaspoon black pepper 

Heat oil in a 12-inch heavy skillet over moderate heat until hot but not smoking, then cook shallots and squash, stirring, until shallots are softened, about 5 minutes. Add broth, brown sugar, sage, and salt, stirring until sugar is dissolved. Simmer, covered, stirring occasionally, until squash is tender, 8 to 10 minutes. Remove from heat and stir in vinegar, pepper, and salt to taste.