Preheat your oven to 375°F. Using a sharp knife, carefully make 6 slits in each chicken breast without cutting all the way through. Drizzle each with some olive oil and season with salt and pepper. Set aside.

Heat ½ tablespoon olive oil in a large ovenproof skillet over medium-high heat. Add spinach and sprinkle with garlic powder. Stir occasionally until spinach is fully wilted, then transfer it to a medium bowl.

To the wilted spinach, add the goat cheese and stir until well combined. Stuff this mixture into the slits of each chicken breast.

In the same skillet, add 1 tablespoon olive oil. Add sliced onions, mushrooms, fresh thyme, optional balsamic vinegar, and season with salt and pepper. Sauté until onions caramelize and mushrooms turn golden brown. Push the veggies to the sides to make space.

Place the stuffed chicken breasts in the skillet with a little space between each. Transfer the skillet to the oven and bake for 20-30 minutes or until the chicken reaches an internal temperature of 165°F.

Remove from oven and serve warm, enjoying the savory stuffed chicken alongside the caramelized onions and mushrooms.