Step 1: Preheat your oven to 375°F (190°C). Prepare the chicken breasts by patting them dry. Lay each breast flat and carefully butterfly it by cutting horizontally almost all the way through, creating a pocket. Pound each breast to an even 1/2-inch thickness.
Step 2: Season both sides of the chicken breasts generously with salt and black pepper.
Step 3: Heat 1 tbsp olive oil in a large skillet over medium heat. Add the finely diced onion and cook until softened, about 3-4 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 4: Add the fresh spinach to the skillet. Cook, stirring occasionally, until the spinach wilts completely, about 2-3 minutes. Remove from heat and stir in the dried cranberries.
Step 5: Divide the spinach and cranberry mixture evenly among the four chicken breasts. Place one slice of Brie cheese on top of the stuffing in each chicken breast.
Step 6: Carefully fold the chicken breast over the filling, enclosing it completely. Secure the edges with 2-3 wooden toothpicks to keep the filling inside.
Step 7: Return the same skillet (or use a fresh oven-safe skillet) to medium-high heat. Add a little more olive oil if needed. Sear the stuffed chicken breasts for 3-4 minutes per side, until golden brown.
Step 8: Deglaze the pan by pouring in the chicken broth or white wine, scraping up any browned bits from the bottom of the skillet. Bring to a simmer.
Step 9: Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the Brie is melted and bubbly.
Step 10: Carefully remove the chicken from the oven. Let it rest for 5 minutes before removing the toothpicks and serving. Serve immediately with your favorite sides.