At Tzuco, in Chicago, the French onion soup arrives to the table piping hot, topped with a bubbling layer of Gruyère cheese. Hiding under the blanket of cheese, a thick slice of baguette soaks up a soup suffused with flavor from a mountain of jammy onions, balsamic vinegar, and bay leaves. French onion soups can lean quite beefy, but chef Carlos Gaytán uses a gentle base of chicken broth that allows the allium flavors to truly shine. Poblano chiles add a mild, vegetal heat and slight charred flavor. To ensure even caramelization, keep the onions in an even layer while they cook.