Now, let’s bring this dish to life with step-by-step instructions:

1. Preheat your oven to 375°F (190°C). Line a baking dish with parchment paper.
2. In a skillet over medium heat, add 1 tablespoon of olive oil. Sauté the onions and garlic until the onions are translucent, about 3 minutes.
3. Add the chopped mushrooms to the skillet and cook until they release their moisture and start to brown, approximately 5 minutes.
4. Stir in the spinach, thyme, oregano, salt, and pepper. Cook until the spinach wilts, about 2 minutes. Remove from heat and let cool slightly.
5. Mix in the mozzarella and Parmesan cheese until well combined.
6. Using a sharp knife, carefully cut a pocket into each chicken breast, being careful not to cut all the way through.
7. Stuff each chicken breast with the mushroom mixture, pressing the edges to seal. If needed, use toothpicks to secure the openings.
8. In a small bowl, combine the remaining olive oil and balsamic vinegar. Brush over the chicken breasts and sprinkle with additional salt and pepper.
9. Place the stuffed chicken breasts in the prepared baking dish. Bake for 25-30 minutes or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).
10. Remove from the oven and let rest for a few minutes before slicing. Garnish with fresh parsley, if desired.

Remember, as I learned from my grandmother, cooking is all about using your senses. Listen to the sizzling sounds, watch for the beautiful golden hues, and enjoy the intoxicating aromas filling your kitchen.