- Turn the heat to medium-low, then add the garlic, pepper, and thyme. Sauté until aromatic.
- Add the balsamic vinegar and beef broth, then stir until well blended. Make sure to scrape the bottom of the skillet to get the browned bits.
- Simmer everything for 10 minutes or until the onions caramelize.
- Add 1/3 of the mixture into the prepared baking dish and spread it evenly.
- Lay the chicken breasts over the onion layer, then season with salt to taste.
- Scatter the rest of the onions over the chicken.
- Sprinkle a generous amount of Parmesan and mozzarella cheese on top of the onions.
- Place the baking dish inside the preheated oven and bake everything for about 30 minutes or until done.
- Remove from the oven and allow it to rest for a few minutes at room temperature.
Serve warm and enjoy!
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