Pat chicken thighs completely dry and season generously on all sides with salt and pepper. Heat olive oil in a large, oven-safe Dutch oven over medium-high heat. Sear the chicken for 3-4 minutes per side until deeply golden brown. Remove from the pot and set aside on a plate.

Reduce heat to medium-low and add the butter to the pot. Once melted, add the sliced onions and a pinch of salt. Cook, stirring occasionally, for 25-30 minutes until the onions are deeply caramelized, jammy, and have a rich amber color.

Add the minced garlic and fresh thyme sprigs to the pot. Cook for one more minute until fragrant.

Pour in the white wine to deglaze the pot, using a wooden spoon to scrape up all the flavorful browned bits from the bottom. Allow the wine to simmer and reduce by about half.

Add the rinsed rice to the pot. Stir constantly for about one minute to toast the grains and coat them in the onion mixture.

Pour in the beef broth, stir everything together, and bring the mixture to a gentle simmer. Taste and adjust seasoning if necessary.

Nestle the seared chicken thighs, along with any accumulated juices from the plate, back into the rice mixture.

Cover the pot with a tight-fitting lid, reduce the heat to low, and let it cook for 18-20 minutes. Do not lift the lid during this time.

After 18-20 minutes, turn off the heat and let the pot stand, still covered, for an additional 10 minutes to allow the rice to finish steaming.

Preheat your broiler. Remove the lid from the pot, discard the thyme sprigs, and gently fluff the rice with a fork.

Sprinkle the shredded Gruyère cheese evenly over the top of the chicken and rice.

Place the pot under the broiler for 2-3 minutes, watching very closely, until the cheese is melted, bubbly, and golden-brown in spots. Let rest for a few minutes before serving.