Use a sharp knife to carefully cut a horizontal slit along the side of each chicken breast, creating a pocket without cutting all the way through.
Place each chicken breast between two sheets of plastic wrap and gently pound with a meat mallet or rolling pin to an even thickness of about 1.5 cm (0.6 inch); this helps it cook evenly and makes stuffing easier.
Sprinkle both sides of the chicken with salt, pepper, and garlic powder for seasoning.
Mix shredded cheese, chopped parsley, thyme, sautéed spinach, sun-dried tomatoes, and toasted breadcrumbs in a bowl to create your stuffing filling.
Using a spoon, gently stuff each chicken pocket with the filling, pressing the edges closed to seal. You can secure with toothpicks if needed.
Heat a tablespoon of olive oil in a skillet over medium-high heat until shimmering and fragrant.
Place the stuffed chicken breasts seam-side down in the hot skillet and sear for 3-4 minutes until the crust is golden and crispy, listening for a satisfying crackle.
Flip the chicken and sear the other side for another 2 minutes, ensuring both sides are beautifully browned.
Transfer the skillet to a preheated oven at 180°C (350°F) and roast for 20-25 minutes, or until a meat thermometer inserted into the center reads 75°C (165°F).
Remove the chicken from the oven, let it rest for 5 minutes to retain juices, then remove toothpicks if used and slice to reveal the cheesy, herby filling inside.
Serve the crispy, juicy stuffed chicken hot, garnished with extra herbs or a squeeze of lemon if desired, and enjoy the delightful contrast of textures and flavors in every bite.
