This jam makes a delicious condiment for any meat, and is the stuffing for our Chicken Breasts Filled With Onion Jam.
Cut onions in half lengthwise; peel. Cut off ends; cut lengthwise into 1/4-inch-thick pieces.
Coat a 13-inch skillet with cooking spray, and set over medium heat. Add onions, and cover. Cook, stirring occasionally, until softened and translucent, about 15 minutes.
Add sugar; re-cover. Cook, stirring occasionally, until onions are deep golden, 20 to 30 minutes. Add 1/4 cup water; stir with a wooden spoon to dislodge any brown bits.
Cover, and cook until dark brown, 20 to 30 minutes. Add balsamic vinegar and another 1/4 cup water. Continue cooking until liquid has been absorbed, about 10 minutes. Transfer to a bowl to cool. Store, refrigerated, in an airtight container up to 1 week.
The cooking time may vary depending on the amount of natural sugar in the onions.