Season the chicken: Pat the chicken dry and season both sides with salt, pepper, onion powder, and garlic powder. This builds flavor right from the start.

Sear the chicken: Heat a large oven-safe skillet over medium-high heat. Add olive oil. Sear chicken 3–4 minutes per side until golden. Remove to a plate. It doesn’t need to be fully cooked yet.

Start the onions: Reduce heat to medium. Add butter to the same pan. Stir in sliced onions and a big pinch of salt. Cook, stirring often, until they soften and start to turn golden, about 10 minutes.

Caramelize fully: Keep cooking the onions 15–20 more minutes, stirring every couple of minutes. Adjust heat as needed to prevent burning. You want a deep, jammy brown. If they look dry, add a splash of water to deglaze and keep them moving.

Add aromatics: Stir in garlic and thyme. Cook 30 seconds until fragrant.

Deglaze: Pour in the white wine (if using). Scrape up browned bits. Let it simmer 2–3 minutes to reduce by about half. If skipping wine, add an extra splash of broth instead.

Make the sauce: Stir in beef broth, Dijon, and balsamic (if using). Simmer 3–5 minutes to thicken slightly. Taste and adjust salt and pepper. The sauce should be savory with a gentle tang.

Nestle the chicken back in: Return chicken and any juices to the skillet, spooning onions over the top. Reduce heat to a low simmer and cook 5–8 minutes, or until the chicken reaches 165°F/74°C.

Add cheese: Sprinkle Gruyère (or Swiss/provolone) over the chicken, then Parmesan. If your skillet is oven-safe, pop it under the broiler 1–2 minutes to melt and lightly brown. Otherwise, cover the pan for a few minutes until melted.

Finish and serve: Garnish with chopped parsley. Serve chicken with a generous spoonful of onion gravy. Pair with cauliflower mash, sautéed green beans, or a crisp salad.