Heat olive oil in a cast iron pan over high heat. Add mushrooms and garlic, cooking for 2-3 minutes until golden brown.

Butterfly the chicken breasts by slicing horizontally almost all the way through. Season with salt and pepper.

On one side of each chicken breast, layer mushroom mixture, spinach, and mozzarella. Fold and secure with a toothpick.

Heat remaining olive oil and butter in the pan over medium-high heat. Sear chicken for 1 minute, then cook for 6-7 minutes per side.