Begin by heating olive oil in a skillet over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant. Then, add the chopped spinach and cook until wilted, about 2-3 minutes. Remove from heat and let it cool slightly before mixing in the shredded mozzarella cheese.
Using a sharp knife, carefully slice each chicken breast horizontally to create a pocket without cutting all the way through.
Divide the spinach and mozzarella mixture evenly among the four chicken breasts. Stuff it inside each pocket and use toothpicks to secure the openings if needed.
Place the stuffed chicken breasts on a baking sheet or in a baking dish. Sprinkle salt, black pepper, and Italian seasoning evenly over the chicken. Finally, sprinkle the grated Parmesan cheese on top.
Preheat your oven to 375°F (190°C). Bake the chicken breasts for 25-30 minutes or until cooked through and cheese is melted. The internal temperature should reach 165°F (74°C).
Let the chicken rest for a few minutes to retain its juices. Remove any toothpicks, then slice and serve hot.