Ingredients

• 2 tablespoons extra-virgin olive oil
• 1 to 2 tablespoons butter (unsalted or salted)
• 2 large yellow onions, thinly and uniformly sliced (about 8 cups), see Tip
• Salt and pepper
• 1 tablespoon balsamic vinegar
• 2 ½ cups/8 ounces shredded Gruyère
• 4 small brioche buns, cut in half crosswise

Directions

• 1. Caramelize the onions: Heat the olive oil and 1 tablespoon butter in a Dutch oven or large, heavy skillet over medium. Once the butter melts, add the sliced onions and a large pinch of salt. (If your onions are particularly large, you may need another tablespoon butter to thoroughly moisten them.) Cook, stirring occasionally, until the onions are softened and translucent, 7 to 10 minutes.
• 2. Reduce the heat to medium-low and continue cooking, stirring occasionally, until the onions are deep brown all over, 1 hour to 1 ¼ hours total, reducing the heat to low if they are browning unevenly. (If the onions start to stick to the bottom of the pan and burn, reduce the heat to low and add the balsamic vinegar, stirring to deglaze the browned bits from the bottom of the pan.) When the onions are caramelized, stir in the balsamic vinegar if you haven’t already added it. Remove from heat.
• 3. Heat the oven to broil (on low, if possible). Place the halved brioche buns open on a sheet pan. Divide the cheese evenly among each bun half and broil until melted, 3 to 4 minutes. Remove the buns from the oven and sprinkle with ground black pepper. Add caramelized onions (about 2 tablespoons) to the bottom half of each bun. Sandwich the two halves and serve.

Nutrition