Preheat and Prepare: First, preheat your oven to 200°C (180°C fan). Get a large, oven-safe frying pan ready. If you don't have one, you'll need a regular frying pan and a baking dish.

Prepare the Chicken: Place a chicken breast on a cutting board. Lay your palm flat on top and, using a sharp knife, carefully slice horizontally into the thickest part of the breast, stopping about 2cm from the other side. Open it up like a book. This is called 'butterflying'. Repeat with the remaining chicken breasts. Season the inside and outside of each breast with salt, pepper, and dried thyme.

Make the Filling: Heat 1/2 tablespoon of the olive oil in the frying pan over a medium heat. Add the minced garlic and cook for 30 seconds until fragrant. Add the fresh spinach and cook for 2-3 minutes, stirring, until it has completely wilted. Remove from the heat and transfer the spinach to a bowl. Squeeze out any excess water once it's cool enough to handle.

Combine the Filling Ingredients: To the bowl with the wilted spinach, add the cubed Brie and dried cranberries. Mix everything together until well combined. The residual heat from the spinach will start to soften the Brie slightly.

Stuff the Chicken: Divide the filling mixture evenly among the four butterflied chicken breasts, spooning it onto one side of the pocket. Fold the other side over to close it. What works best for me is using two or three cocktail sticks to secure the opening and keep that delicious filling from escaping during cooking.

Sear the Chicken: Return your oven-safe frying pan to a medium-high heat and add the remaining 1/2 tablespoon of olive oil. Carefully place the stuffed chicken breasts in the hot pan. Sear for 3-4 minutes on each side, until they are beautifully golden brown. Do this in batches if needed to avoid overcrowding.

Bake to Perfection: Transfer the oven-safe pan directly into the preheated oven. If you're not using an oven-safe pan, move the seared chicken to a baking dish. Bake for 15-20 minutes, or until the chicken is cooked through and the cheese is melted and bubbly. The internal temperature should reach 74°C on a meat thermometer, as recommended by the Food Standards Agency.

Rest and Serve: Remove the chicken from the oven and let it rest on a cutting board for 5 minutes. This is a crucial step to ensure the juices redistribute for a tender result. Don't forget to remove the cocktail sticks before slicing and serving!