In a skillet, heat a drizzle of olive oil over medium heat. Add the finely diced onion and sauté until it becomes translucent. Pour in the broth and add the baby spinach. Stir frequently until the spinach has wilted and all the liquid has completely evaporated. This ensures the filling stays inside the chicken and doesn't make the dish soggy. Remove from heat and set aside.

Place the chicken breasts on a cutting board. Using a sharp knife, slice each breast horizontally through the middle, being careful not to cut all the way through. Open them up like a book. Season both the inside and outside with salt and black pepper.

Spread the sautéed spinach mixture evenly inside each breast, then add 2 teaspoons of cream cheese to each. Fold the chicken back over the filling and secure the edges with 2 or 3 toothpicks to keep everything contained.

Preheat your oven to 350°F (180°C). Place the stuffed breasts in a baking dish and bake for approximately 30 minutes. It is important to watch the time so the chicken remains juicy and does not dry out.

Remove the dish from the oven and sprinkle the shredded cheese over the top of each breast. Return to the oven (or switch to the broiler) for about 5 minutes, until the cheese is bubbly and golden brown.

Remember to remove the toothpicks before serving. This dish pairs beautifully with a fresh arugula salad or a side of steamed vegetables.