Preheat the oven to 400°F (204°C).
Slice the chicken breasts in half lengthwise to create four thinner fillets. Arrange in a baking dish and season with salt and pepper. Sprinkle half of the minced garlic over the top.
In a mixing bowl, combine softened cream cheese and heavy cream. Beat until smooth. Incorporate remaining garlic, onion powder, paprika, and Italian seasoning. Add grated Parmesan and half of the mozzarella cheese, mixing thoroughly.
Spread the creamy mixture over the seasoned chicken fillets. Drizzle with reserved oil from sun-dried tomatoes and scatter chopped tomatoes around the dish.
Cover the baking dish loosely with aluminum foil and bake for about 20 minutes or until chicken reaches an internal temperature of 165°F (74°C).
Stir coarsely chopped spinach into the sauce surrounding the chicken and sprinkle remaining mozzarella cheese on top.
Return the baking dish to the oven uncovered for an additional 10 minutes until cheese melts and spinach wilts.
Let the casserole rest briefly before serving. Serve hot alongside crusty bread or a fresh salad.