Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch casserole dish and set aside.
Cook the chicken: Heat olive oil in a large skillet over medium-high heat. Add the chicken pieces, season with salt, pepper, paprika, and Italian seasoning, and cook until golden brown on all sides (about 5-7 minutes). Remove the chicken and set aside.
Make the sauce: In the same skillet, melt the butter over medium heat. Add the garlic and sauté for about 1 minute until fragrant. Stir in the flour and cook for another minute, creating a roux. Slowly whisk in the chicken broth and heavy cream, stirring constantly until the mixture thickens (about 3-4 minutes).
Add the vegetables: Stir in the spinach and sun-dried tomatoes into the sauce, cooking for another 2 minutes until the spinach wilts.
Combine the ingredients: Add the cooked chicken back into the skillet and stir to coat in the sauce. Pour the mixture into the prepared casserole dish, spreading it evenly.
Top with cheese: Sprinkle the mozzarella and parmesan cheeses evenly over the casserole.
Bake: Place the casserole in the oven and bake for 20-25 minutes, or until the cheese is melted and bubbly.
Serve: Remove from the oven and let the casserole rest for 5 minutes before serving. Garnish with fresh basil, if desired.
