3 tablespoons butter
2 large red onions, sliced
2 tablespoons sugar
3 tablespoons balsamic vinegar

Heat butter in large skillet over medium flame. Stir in onions and sugar and cook, stirring frequently, until onions are caramelized, about 20 minutes. De-glaze the pan, i.e. stir vinegar into pan to release any bits from bottom of pan and then cook until all the vinegar has been absorbed into the onion mix and the flavors come together, about another 7 minutes. This makes a delicious topping for grilled fish, meats, chicken or tofu.

source: From Asparagus to Zucchini