Use a sharp boning knife to carefully slice a horizontal pocket into each chicken breast, creating a space for the stuffing without cutting all the way through.
In a small bowl, combine the caramelized onions, wilted spinach, crumbled goat cheese, minced garlic, chopped parsley, and a pinch of salt and pepper. Mix until well combined.
Gently stuff each chicken pocket with the mixture, pressing lightly to pack it in, then seal the opening with toothpicks to prevent spilling during cooking.
Heat a skillet over medium-high heat, adding a tablespoon of olive oil. Once shimmering, sear the stuffed chicken breasts for about 2-3 minutes per side until they develop a golden-brown crust and smell deliciously nutty.
Preheat your oven to 190°C (375°F). Transfer the skillet with the chicken to the oven and bake for 20-25 minutes, or until the internal temperature reaches 74°C (165°F) and the juices run clear.
Remove the skillet from the oven, let the chicken rest for 5 minutes, then carefully remove the toothpicks and slice each breast to reveal the melty, savory filling inside. Serve hot and enjoy the beautiful, flavorful result.
