Preheat the oven to 375°F (190°C). Pat the chicken breasts dry with paper towels and set aside.
In a mixing bowl, combine the chopped spinach, cream cheese, mozzarella cheese, minced garlic, parsley, basil, thyme, black pepper, salt, and red pepper flakes (if using). Mix until well combined and creamy.
Carefully butterfly each chicken breast by slicing it horizontally, ensuring not to cut all the way through.
Generously spoon the herb and cheese filling into each butterflied chicken breast. Fold the chicken over to enclose the filling and secure with toothpicks.
In a large oven-safe skillet, heat the olive oil over medium-high heat. Add the stuffed chicken breasts and sear for about 3-4 minutes on each side until golden brown.
Carefully pour the chicken broth into the skillet around the chicken. Bring it to a gentle simmer, then transfer the skillet to the preheated oven. Bake for 25-30 minutes or until the chicken is cooked through.
Once baked, remove the chicken from the oven and let it rest for about 5 minutes. Discard toothpicks. Serve hot with lemon wedges on the side.
Notes
Serve with lemon wedges and a side of roasted vegetables or salad.