Preheat your oven to 375F and move the rack to the middle position.
Cut a pocket into each chicken breast, taking care not to cut through the other side. See the step-by-step photos in the blog post if you're unsure of where to cut.
Generously season the chicken with salt & pepper.
Add one tablespoon of the olive oil to a large oven-safe skillet (I use my 12" Le Creuset) over medium heat, and sauté the onion until soft and lightly browned (about 5-7 minutes).
Stir in the garlic and cook for 30 seconds.
Add in the spinach and cook for a few minutes until wilted (stir/toss often), then take the skillet off the heat.
In a medium prep bowl, combine the Boursin, mozzarella, Italian seasoning, crushed red pepper flakes, and spinach mixture from the skillet. Do not wash the skillet yet - you will use it to cook the chicken.
Using a spoon, stuff the chicken breasts evenly with the filling. I use 2 toothpicks in each chicken breast to close them. Don't worry if you don't do this perfectly. Some of the filling will spill out in the next step and that's ok - it helps makes a tasty pan sauce.
Add the remaining olive oil to the skillet and heat it up over medium-high heat for a few minutes. Once the pan is hot, add the chicken and cook for 5 minutes on the first side, flip using tongs, and cook for another 3 minutes.
Transfer the skillet to the oven and bake, uncovered, for 15 minutes or until the chicken is cooked through. Rest for 5 minutes prior to serving. The pan juices are delicious over mashed potatoes or rice. Don't forget to remove the toothpicks prior to serving!