Make the Mushroom Spinach Filling - Melt the butter in a large ovenproof skillet over medium heat. Add the sliced mushrooms and cook for about 5 minutes until they turn golden brown on the edges. Add garlic and cook for one more minute.
Reduce the heat and mix in cream cheese and heavy cream. Stir to combine and season with thyme, salt, and pepper to taste.
Add baby spinach and cook for about 30 seconds until wilted.
Cut a pocket into each chicken breast - Using a sharp knife, slice through the chicken breast (see video), creating a pocket.
Season the chicken breast with salt and pepper to taste.
Stuff the chicken breast with 2-3 tablespoons of filling, place 2 slices of mozzarella, and seal with toothpicks.
Sear chicken: Clean the skillet with paper towels and add olive oil. Sear each side of the chicken breast for 3 minutes on each side until golden. Top with the remaining filling mixture(if any!).
Bake at 200°C/390°F for 15-20 minutes or until the chicken is cooked through. The internal temperature of the chicken should be 149°F/ 65°C when inserted in the thickest part.
Remove from the oven and allow to rest 5 minutes. Enjoy!