Cook the spinach and garlic in a skillet until softened, then let it cool slightly. Combine with cream cheese, mozzarella, Parmesan, sun-dried tomatoes, and herbs in a bowl. Mix until creamy and well incorporated.
Slice a pocket into the side of each chicken breast, being careful not to cut all the way through. Fill each pocket with the prepared mixture, then season the outside with salt, pepper, and paprika.
Heat oil in a skillet over medium-high heat. Sear each breast for 2-3 minutes per side until golden brown. This locks in the juices and adds flavor.
Transfer the skillet to a 375°F oven and bake for 17 minutes, or until the thickest part of the chicken reaches 165°F. Let it rest for a few minutes before slicing to keep it juicy.