Preheat the oven to 400°F (200°C).
Using a sharp knife, carefully cut a pocket into the thickest side of each chicken breast without slicing all the way through.
Stuff each chicken breast with baby spinach, sun-dried tomatoes, and a slice of fresh mozzarella.
Season the outside of each breast with Italian seasoning, kosher salt, and black pepper.
Secure the openings with toothpicks if needed.
Heat the reserved sun-dried tomato oil in a large oven-safe skillet over medium-high heat.
Sear the chicken for about 3 minutes per side, until golden brown.
Transfer the skillet to the oven and bake the Caprese Stuffed Chicken for approximately 15 minutes, or until the internal temperature reaches 165°F (74°C).
Remove from the oven and let rest for 5 minutes.
Remove toothpicks and drizzle the pan juices over the chicken before serving.