01 - Preheat the oven to 400°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Season chicken breasts generously with salt, pepper, and Italian seasoning on both sides. Arrange them in a single layer in the prepared baking dish.
03 - In a medium bowl, whisk together heavy cream, grated Parmesan, minced garlic, and red pepper flakes until well combined.
04 - Pour the cream mixture evenly over the chicken breasts. Scatter sun-dried tomatoes and chopped spinach around and between the chicken pieces. Top everything with shredded mozzarella cheese.
05 - Bake for 25-30 minutes until the chicken is cooked through (internal temperature reaches 165°F) and the cheese is golden and bubbly.
06 - Remove from the oven and let the casserole rest for 5 minutes to allow the sauce to thicken slightly. Garnish with fresh basil if desired and serve hot.