Preheat the oven to 375°F (190°C). Pat the 4 chicken breasts dry with paper towels.
Cut a horizontal slit into the side of each chicken breast to form a pocket, being careful not to cut through the opposite side.
Stuff each breast by evenly dividing the filling: place 1 slice of mozzarella inside each pocket, and distribute the ½ cup sliced sun-dried tomatoes and 2 cups spinach evenly among the 4 breasts so each pocket is filled. Close each pocket and secure with 1–2 toothpicks placed diagonally.
Rub both sides of the stuffed breasts with half of the 2 tablespoons olive oil (1 tablespoon) and sprinkle the 1 tablespoon Italian seasoning evenly over all sides.
Heat the remaining 1 tablespoon olive oil in a large oven-safe skillet over medium-high heat. Add the stuffed chicken and cook 2–3 minutes per side, until each side is golden brown.
In a small bowl, whisk together the ½ cup heavy cream, ¼ cup chicken broth, ½ teaspoon garlic powder, 1 teaspoon Italian seasoning, and ¼ cup parmesan cheese. Pour this sauce into the bottom of the skillet around the chicken.
Transfer the skillet to the preheated oven and bake 20 minutes, or until the cheese is bubbly and the chicken is cooked through (no longer pink in the center or reaches 165°F). Remove toothpicks before serving.