Preheat your oven to 375°F (190°C).
In a large oven-safe skillet, heat the olive oil over medium heat. Add the sliced onions and sprinkle with salt. Cook, stirring occasionally, for about 15-20 minutes until the onions are caramelized and golden brown.
While the onions are cooking, season the chicken breasts with black pepper and dried thyme.
Once the onions are caramelized, push them to the side of the skillet and add the chicken breasts to the center. Sear the chicken for about 3-4 minutes on each side until lightly browned.
Pour the beef broth over the chicken and onions, ensuring the chicken is mostly submerged.
Sprinkle the shredded Swiss cheese evenly over the top of the chicken and onions.
If using, add the cubed baguette on top of the cheese for a crunchy topping.
Transfer the skillet to the preheated oven and bake for 25-30 minutes, or until the chicken is cooked through (internal temperature of 165°F or 74°C).
Remove from the oven and let it rest for a few minutes. Garnish with fresh parsley before serving.