Start by seasoning the chicken thighs with salt and pepper. Heat olive oil in a skillet over medium-high heat and sear the chicken until golden brown on both sides. Remove from the pan and set aside while you cook the onions.
Reduce the heat to medium and add sliced onions to the same skillet. Stir occasionally, letting them cook slowly until deep golden brown. Add garlic and thyme in the last 2 minutes to release their aroma.
Add uncooked rice to the skillet and stir to coat with onion mixture. Pour in chicken broth and Worcestershire sauce, then return the chicken to the pan. Bring to a simmer, cover, and cook until the rice is tender and chicken is fully cooked.
Once the rice is done, sprinkle shredded Gruyère or Swiss cheese over the top. Cover the pan for a few minutes to let the cheese melt perfectly. Serve immediately for a warm, comforting meal.