Preheat your oven to 375°F (190°C).
In a large skillet over medium heat, add the olive oil. Once hot, add the minced garlic and sauté for about 1 minute until fragrant.
Add the fresh spinach to the skillet and cook until wilted, about 2-3 minutes. Remove from heat and let it cool slightly.
In a mixing bowl, combine the cooked spinach, ricotta cheese, Parmesan cheese, mozzarella cheese, salt, black pepper, red pepper flakes, and Italian herbs. Mix well to create the stuffing.
Slice a pocket into each chicken breast by cutting horizontally, but not all the way through.
Stuff each chicken breast with the spinach and cheese mixture, then secure the openings with toothpicks or tie them with kitchen twine to hold the filling inside.
Place the stuffed chicken breasts in a baking dish.
Bake in the preheated oven for 25-30 minutes, or until the internal temperature reaches 165°F (75°C) and the chicken is cooked through.
Allow the chicken to rest for 5 minutes before removing the toothpicks or twine. Slice and serve.