Part 5/9:
The onions are drained to remove excess water, and a small amount of lightly browned butter infused with extra virgin olive oil is heated in a skillet. The drained onions are added to this skillet, and the process of caramelization begins.
A crucial aspect here is building layers of flavor: as the onions cook, balsamic vinegar and a touch of light brown sugar are added. The vinegar dissolves into the onions, creating a beautiful, glossy glaze, while the sugar lends sweetness and helps develop a deep caramel color. The chef explains this balance is vital to avoid overly sweet or sour results, emphasizing that a good gastrique (French for sweet and sour sauce) balances these elements perfectly.