Preheating the Oven: Commence by preheating the oven to 400°F (204°C) to ensure it reaches the correct temperature for baking.
Preparing the Chicken: Take the chicken breasts and slice them in half lengthwise to create four thinner fillets. Arrange these fillets in a single layer within a baking dish. Season them liberally with kosher salt and freshly ground black pepper, and sprinkle half of the minced garlic over the top. Set aside momentarily.
Creating the Sauce: In a medium mixing bowl, combine the softened cream cheese with the heavy cream. Using an electric mixer or a whisk, beat these ingredients together until the mixture is smooth and homogeneous. Incorporate the remaining minced garlic, onion powder, paprika, and Italian seasoning into the cream cheese mixture. Season additionally with salt and pepper to taste. Add the grated Parmesan cheese and half of the mozzarella cheese to the bowl and beat again until all components are fully integrated.
Assembling the Casserole: Evenly spread the cream cheese mixture over and around the seasoned chicken fillets in the baking dish. Drizzle approximately one tablespoon of the reserved oil from the sun-dried tomatoes over the mixture. Scatter the chopped sun-dried tomatoes evenly around the dish.
Initial Baking: Cover the baking dish loosely with aluminum foil to retain moisture and prevent the surface from burning. Place the dish in the preheated oven and bake for 20 minutes, or until the chicken reaches an internal temperature of 165°F (74°C), indicative of being thoroughly cooked.
Adding Final Ingredients: Remove the dish from the oven, and carefully uncover it. Stir the coarsely chopped spinach into the sauce around the chicken and sprinkle the remaining mozzarella cheese over the top.
Final Baking: Return the baking dish to the oven, uncovered, and continue to bake for an additional 10 minutes, or until the cheese has melted completely and the spinach has wilted.
Serving: Remove the casserole from the oven and let it rest briefly before serving to allow the flavors to meld together further. Serve hot.