Preheat oven to 375°F (190°C). Grease a 9x13 inch baking dish with cooking spray or butter.
Cook the chicken: Heat olive oil in a large skillet over medium-high heat. Season chicken pieces with salt, pepper, and paprika. Cook until golden and cooked through, 6-8 minutes. Remove and set aside.
Sauté vegetables: In the same skillet, add diced onion and cook until softened, about 4 minutes. Add minced garlic and cook for 1 minute until fragrant.
Wilt spinach: Add chopped spinach to the skillet and stir until wilted, 2-3 minutes. The residual heat will do most of the work.
Make cream sauce: Reduce heat to low. Add softened cream cheese and heavy cream. Stir constantly until cream cheese melts completely and creates a smooth sauce.
Combine: Return cooked chicken to the skillet. Add Italian seasoning and stir everything together until well coated. If using rice, mix it in now.
Assemble casserole: Transfer mixture to prepared baking dish and spread evenly. Top with 1 1/2 cups mozzarella and all Parmesan cheese.
Bake: Bake for 15-20 minutes until cheese is melted and bubbly. For golden crust, broil for final 2-3 minutes, watching carefully.
Serve: Let rest for 5 minutes. Garnish with fresh parsley and serve hot.