Preheat the oven to 425°F.
Prep the chicken: Place the chicken breasts between two pieces of parchment paper and gently pound the centers to a more even thickness. You don't want to pound them thin, just enough to help them cook evenly and give you more surface area for stuffing. Pat completely dry with paper towels.
1 1/2 lbs boneless skinless chicken breasts
Cut the pockets: Place each chicken breast flat on a cutting board. Starting at the center of the thick side, cut a pocket almost all the way through, leaving about 3/4 inch on each end. Think pita pocket, not butterfly, you're making a pouch, not opening the breast flat.
Make the filling: Squeeze as much moisture as you can out of the thawed spinach with a paper towel. Add it to a medium bowl with the mozzarella, Italian seasoning, garlic powder, and Boursin. Mix until combined. The filling should be thick and hold its shape.
1/4 cup frozen spinach, 1 cup low-moisture shredded mozzarella, 1 teaspoon Italian seasoning, 1 teaspoon garlic powder, 1 (5oz) wheel garlic herb Boursin cheese
Stuff and seal: Spoon the filling evenly into each pocket. Don't overfill. Push two toothpicks through both sides of the opening to close the seam. Season the outside generously with salt and pepper.
1 teaspoon kosher salt, Black pepper to taste
Sear the chicken: Heat the olive oil in a large oven-safe skillet over medium-high heat until shimmering. Place the chicken top-side down and cook for five minutes without moving it, until deeply golden brown. Flip with tongs and sear the second side for two to three minutes.
1 Tablespoon olive oil
Finish in the oven: Carefully transfer the pan to the oven and bake for 10 to 12 minutes, until the internal temperature reaches 165°F. Rest for five minutes before serving. Remove toothpicks before plating.