Start by slicing the onions thinly and seasoning the chicken with salt and pepper. I like to have everything ready before turning on the stove because once the onions start cooking, they need attention. Preheating the oven early also saves time later.
Heat olive oil and butter in your skillet over medium heat and add the onions. Cook them slowly, stirring often, until they turn golden brown and soft. This step takes patience, but it’s where most of the flavor comes from.
Once the onions are deeply caramelized, stir in the garlic and thyme. Let them cook for about 30 seconds until fragrant. This short step wakes up the herbs and prevents the garlic from burning.
Stir the rice into the onion mixture so it gets coated in all that flavor. Pour in the beef broth and gently scrape the bottom of the pan. Bring everything to a light simmer so the rice can start absorbing liquid.
Place the seasoned chicken thighs on top of the rice mixture. Press them down slightly so they’re partially submerged. This helps the chicken stay moist while baking.
Cover the skillet tightly with foil and transfer it to the oven. Bake until the rice is tender and the chicken is fully cooked. The steam trapped inside ensures even cooking.
Remove the foil, sprinkle grated Gruyère over the top, and return the skillet to the oven uncovered. Bake just until the cheese melts and becomes lightly golden. Let it rest for a few minutes before serving.