Preheat your oven to 375°F (190°C).

In a large, oven-safe skillet or Dutch oven, melt the butter over medium-low heat. Add the sliced onions and a generous pinch of salt. Cook slowly, stirring occasionally, for 30-40 minutes until the onions are a deep, jammy, golden-brown color. Add the minced garlic and cook for one more minute until fragrant. Remove the onion mixture from the skillet and set aside.

While the onions are cooking, pat the chicken thighs dry and season them generously on both sides with salt and pepper.

Increase the heat in the same skillet to medium-high. Add the olive oil. Place the chicken thighs in the hot pan and sear for 3-4 minutes per side until a golden-brown crust forms. The chicken does not need to be cooked through. Remove the chicken and set it aside with the onions.

Lower the heat back to medium. Pour the white wine into the skillet to deglaze, using a wooden spoon to scrape up all the browned bits from the bottom of the pan. Let the wine bubble and reduce by about half.

Pour in the beef broth, Worcestershire sauce, and fresh thyme. Bring the mixture to a simmer. Stir in the uncooked rice, making sure it's fully submerged in the liquid.

Return the caramelized onions to the skillet and stir them into the rice mixture. Nestle the seared chicken thighs on top of the rice. Bring the entire mixture back to a gentle simmer on the stovetop.

Cover the skillet tightly with a lid or aluminum foil and transfer it to the preheated oven. Bake for 25 minutes.

After 25 minutes, carefully remove the cover. Sprinkle the shredded Gruyère and Provolone cheeses evenly over the top.

Return the skillet to the oven, uncovered, and bake for another 10-15 minutes, or until the rice is tender, the chicken is cooked through (reaches 165°F or 74°C), and the cheese is gloriously melted, bubbly, and golden brown.

Let the bake rest for 5-10 minutes before serving.