Preheat your oven to 375°F (190°C).
In a large skillet, heat olive oil over medium heat. Add garlic and onion, sautéing until translucent. Add mushrooms, and cook until they release their moisture. Stir in spinach, cooking until wilted. Season with thyme, salt, and pepper. Set aside to cool slightly.
Place chicken breasts on a cutting board. Make a horizontal cut along the side of each breast to create a pocket. Be careful not to cut all the way through.
Divide the mushroom and spinach mixture into four portions. Stuff each chicken breast with the mixture. If desired, add a sprinkle of low-fat cheese into each pocket. Secure the openings with toothpicks or kitchen twine.
Season the outside of the chicken breasts with salt and pepper.
Place the stuffed chicken breasts on a baking tray lined with parchment paper. Bake in the preheated oven for 25-30 minutes, or until the chicken is cooked through and no longer pink in the middle.
Let the chicken rest for a few minutes before serving. Remove toothpicks or twine before eating.
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.