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French Onion Sliders

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Ingredients
  

  • 2 tbsp. extra-virgin olive oil
  • 1 to 2 tbsp. salted or unsalted butter
  • 8 cups yellow onions, thinly and uniformly sliced (see note)
  • Salt and pepper, to taste
  • 1 tbsp. balsamic vinegar
  • 2-½ cups Gruyère, shredded
  • 4 small brioche buns, cut in half crosswise

Instructions
 

  • In a Dutch oven or large, heavy skillet, over medium heat, heat the olive oil and 1 tablespoon butter.
  • Once the butter melts, add the sliced onions and a large pinch of salt; if the onions are particularly large, another tablespoon of butter may be needed to thoroughly moisten.
  • Cook, stirring occasionally, until the onions are softened and translucent; 7 to 10 minutes.
  • 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/or 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.
  • Heat the oven to broiler on low.
  • Place the halved brioche buns, opened, 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 2 tablespoons or caramelized onions to the bottom half of each bun.
  • Complete the sandwich by placing the two halves together and serve.

Notes

Onions that are sliced uniformly will brown more evenly; onions that are sliced thinly will caramelize faster.