Prep: Preheat the oven to 430°F (220°C). Get out a 9x13 casserole dish and a large piece of foil to cover it with.
Assemble: Place chicken, Italian seasoning, garlic and pasta into the dish. Whisk together stock, milk and cornstarch in a large measuring jug. Pour into the casserole dish and stir.
Bake: Cover the dish with foil and bake for 30 minutes.
Finish assembling: Remove the dish from the oven and very carefully remove the foil. Stir in the sun dried tomatoes, both cheeses and the spinach. Bake for 10-15 minutes, uncovered, until bubbly. Serve immediately.