Fill store-bought brioche buns with jammy, caramelized onions and melted Gruyère, for satisfying little sandwiches that make a perfect soup companion or a cozy, comforting meal. Caramelizing the onions takes some time but very little effort, and coaxing out the ingredient’s sweet side is well worth it. The butter used to caramelize the onions adds flavor, and the extra-virgin olive oil allows the onions to tolerate higher heat without burning. (However, if you find the onions are browning unevenly, turn the heat down to low.) Prepare a big batch of these balsamic caramelized onions ahead of time and lunch can be on your plate in 10 minutes.