Prepare the Oven and Chicken: Preheat your oven to 200°C (180°C fan). Pat the chicken breasts completely dry with a paper towel. This is crucial for getting a good sear. Using a sharp knife, carefully slice a deep pocket into the thickest side of each chicken breast, being careful not to cut all the way through. Season the inside and outside of each breast generously with salt and pepper.

Make the Cheese & Spinach Filling: In a medium-sized, oven-safe skillet, melt 1 tablespoon of butter over medium heat. Add the minced garlic and cook for about 30 seconds until fragrant. Add the fresh spinach in batches, stirring until it has completely wilted down. Squeeze out as much excess water from the spinach as possible and chop it finely.

Combine the Filling Ingredients: In a bowl, mix the softened cream cheese, grated cheddar, grated Parmesan, chopped spinach, 1/4 tsp salt, 1/4 tsp pepper, and the onion powder. Stir until everything is well combined.

Stuff the Chicken: Carefully divide the cheese mixture among the four chicken breasts, spooning it into the pockets you created. Don't overfill them, as the cheese will expand and ooze out during cooking. I find that using a few toothpicks to seal the opening works wonders to keep most of the delicious filling inside.

Sear the Chicken: Return your oven-safe skillet to a medium-high heat. Add the olive oil and the remaining tablespoon of butter. Once the butter is melted and sizzling, carefully place the stuffed chicken breasts in the pan. Sear for 3-4 minutes per side, until they are a deep golden brown.

Bake to Perfection: Transfer the entire skillet to the preheated oven. Bake for 15-20 minutes, or until the chicken is cooked through. The most reliable way to check is with a meat thermometer; it should read 74°C at the thickest part of the chicken. For more detailed information on cooking chicken safely, you can consult the Food Standards Agency guidelines.

Create the Pan Sauce: Remove the chicken from the skillet and let it rest on a cutting board. Place the skillet back on the hob over medium heat. Pour in the white wine to deglaze the pan, scraping up all the tasty browned bits from the bottom. Let it bubble and reduce by about half, which takes 1-2 minutes.

Finish the Sauce and Serve: Pour in the chicken stock and let it simmer for another 2-3 minutes until the sauce has thickened slightly. Remove from the heat. Slice the chicken and serve immediately with the warm pan sauce drizzled over the top and a sprinkle of fresh parsley.