Start by melting butter in a large skillet over medium-high heat. Add the sliced mushrooms and cook until golden and tender, about 5 minutes. Stir in garlic and thyme for a minute, releasing their fragrant oils. Remove from heat and set aside.

After cooking the mushrooms, toss the baby spinach into the skillet. Stir for about 30 seconds until just wilted. Remove from the skillet, combine the mushrooms and spinach, and allow the mixture to cool slightly.

Take your chicken breasts and, using a sharp knife, slice carefully to create a pocket in each. Be sure not to cut all the way through. Lightly season both the inside and outside with salt and pepper for flavor.

Once the filling has cooled, carefully stuff each chicken pocket with the mushroom-spinach mixture. Add mozzarella slices to each portion, tucking them into the pocket. Secure the opening with toothpicks to prevent the filling from spilling out.

Heat olive oil in a skillet over medium heat. Sear the stuffed chicken breasts for 1–2 minutes on each side until golden brown. This step locks in the juices and creates a delicious crust.

Transfer the seared chicken to an oven preheated to 390°F (200°C). Bake for 15 minutes or until the chicken reaches an internal temperature of 165°F. Allow the chicken to rest for 5 minutes before serving to lock in the juices.