Preheat your oven to 375°F. Make 6 slits in each chicken breast without cutting all the way through. Drizzle each with 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 until spinach is fully wilted, then transfer it to a medium bowl.

Add goat cheese to the wilted spinach 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 onions, mushrooms, fresh thyme, balsamic vinegar, and season with salt and pepper. Sauté until onions caramelize and mushrooms turn golden brown, then push veggies to the sides.

Place the stuffed chicken breasts in the skillet with 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 alongside the caramelized onions and mushrooms.