Preheat your oven to 190°C (375°F). Gather a chef's knife, a meat mallet, and a skillet. Pound each chicken breast to an even thickness of about 1.5 cm (0.6 inches) to ensure uniform cooking.
Use a sharp knife to carefully slice a horizontal pocket into each chicken breast, creating a space for the filling without cutting all the way through. Season both sides generously with salt and pepper.
In a bowl, combine chopped spinach, minced garlic, shredded cheese, and chopped herbs. Mix well until evenly distributed.
Stuff each pocket with the cheese and herb mixture, pressing gently to pack it in and ensure it stays inside during cooking.
If needed, secure the openings with toothpicks to prevent fillings from spilling out during searing and roasting.
Brush the stuffed chicken breasts lightly with olive oil to promote browning. Heat a skillet over medium-high heat and sear each side for about 3-4 minutes, until golden brown and crispy.
Transfer the skillet to the preheated oven or move the chicken to an oven-safe dish. Roast for 20-25 minutes, or until the internal temperature reaches 75°C (165°F) and the cheese inside is melting.
Remove the skillet from the oven and let the chicken rest for 5 minutes. Carefully remove toothpicks and slice the chicken diagonally to showcase the gooey filling.
Serve the stuffed chicken breasts warm, optionally drizzled with pan juices or accompanied by a fresh salad for a complete meal.