• Caramelize the onions

  Heat a large cast iron skillet over medium-low heat. Add butter and olive oil.

  Add sliced onions and a generous pinch of kosher salt. Stir to separate and coat.

  Cook low and slow, stirring occasionally, for 20–40 minutes until deeply golden and caramelized.

  If the pan becomes dry near the end, add 1 teaspoon balsamic vinegar to loosen and deepen flavor.

  Remove onions from the skillet and set aside.

  Season the chicken

  In a bowl, add chicken thighs, olive oil, balsamic vinegar, dried thyme, salt, and black pepper.

  Mix well using your hands to evenly coat. Set aside.

  Sear the chicken

  Increase skillet heat to medium-high. Add olive oil or avocado oil.

  Place chicken skin-side down and sear about 3 minutes per side until lightly golden.

  Remove chicken from the skillet.

  Build the sauce

  Reduce heat to medium. Add butter to the skillet.

  Whisk in flour and cook briefly to form a roux.

  Deglaze with red wine, scraping up browned bits. Let reduce slightly.

  Whisk in beef broth, heavy cream, mustard, and Worcestershire sauce.

  Combine and bake

  Stir the caramelized onions and any accumulated juices into the sauce.

  Return chicken to the skillet, nestling it into the onion mixture.

  Turn off the stovetop heat.

  Transfer to a 425°F oven and bake for about 20 minutes, checking internal temperature on larger thighs.

  Flip or spoon sauce over chicken once if desired.

  Add cheese and broil

  Remove skillet from oven. Snuggle chicken toward the center if needed.

  Top generously with grated Gruyère.

  Switch oven to broil and return skillet for 3–5 minutes, watching closely, until cheese is melted and bubbly.

Notes

• Beef broth gives the most traditional French onion flavor, but chicken broth works well if needed.

  Gruyère can be swapped for Swiss, though Gruyère offers the best melt and flavor.

  Bone-in, skin-on thighs are ideal, but boneless thighs can be used with a shorter cooking time.