Preheat your oven to 375°F (190°C).
Prepare the filling by combining spinach, mozzarella, cream cheese, garlic, salt, pepper, and Italian seasoning in a bowl.
Cut a pocket in each chicken breast, being careful not to slice all the way through.
Stuff each chicken breast with the spinach and mozzarella mixture, securing with toothpicks if necessary.
Season the outside of the chicken breasts with salt, pepper, and red pepper flakes if using.
Heat olive oil in a skillet over medium-high heat.
Sear the stuffed chicken breasts for about 4-5 minutes on each side until golden brown.
Transfer the chicken breasts to a baking dish and bake for 20-25 minutes.
Let the chicken rest for 5-10 minutes before slicing.
Slice the stuffed chicken breasts and serve them warm.