Preheat your oven to 400°F (200°C) and spray a baking dish with nonstick cooking spray.

Season the chicken breasts generously with salt and pepper. In a large skillet over medium heat, melt the butter, then add the sliced onions. Sauté these until they turn golden brown, about 15-20 minutes.

Add minced garlic to the skillet and stir it in, followed by the rice. Mix everything together until combined and fragrant.

Pour in the chicken broth, soy sauce, and balsamic vinegar. Bring to a boil, then reduce heat to low, cover, and simmer for around 20 minutes until the rice is tender.

Transfer the mixture to your prepared baking dish. Sprinkle the shredded Gruyère cheese generously on top. Bake for an additional 10 minutes, or until the cheese is bubbly and golden brown.