Preheat the oven to 200°C conventional oven/ 180°C fan-forced (400° Fahrenheit/Gas 6). Line a baking tray with baking paper.
Wipe the pumpkin to remove any exterior dirt. Use a sharp knife to carefully cut the top off the pumpkin and set aside. Use a knife to cut out or a pumpkin scraper (or large spoon) to scrape out the internal seeds and stringy parts of the pumpkin and discard. Scrape out the pumpkin flesh, leaving a 2-3cm border around the walls of the pumpkin and transfer to a bowl.
Lay a large sheet of aluminium foil onto a separate baking tray. Place the whole pumpkin on top of the foil. Wrap the pumpkin lid in a separate sheet of aluminium foil to enclose and place to the side of the pumpkin. Bake the pumpkin, uncovered for 30 minutes.
Meanwhile, cut the scooped out pumpkin flesh and chop into small cubes. Place the onion, chopped pumpkin and sage into the baking-paper lined baking tray. Drizzle with olive oil and season with sea salt and freshly ground black pepper. Bake for 20-30 minutes or until the pumpkin is tender.
To make the Besciamella sauce, melt the butter in a medium-sized saucepan over medium heat until foaming. Add the flour and stir with a wooden spoon to form a smooth paste (a roux). Allow the flour and butter mixture to cook for a minute (but do not allow it to brown). Remove from the heat. Slowly pour the cold milk into the roux, stirring constantly, until the mixture is smooth. Add the bay leaf and return to medium heat. Cook, stirring with the wooden spoon until the Besciamella comes to the boil and thickens (approximately 10 minutes). Remove from heat. Season with salt to taste and stir in freshly grated nutmeg. Set aside to cool slightly.
Allow the whole pumpkin and roasted chopped pumpkin to cool slightly. Remove ¼ of the pumpkin pieces and set aside. Blend the remaining roasted pumpkin pieces together with red onion and sage. Transfer the blended pumpkin and reserved pumpkin pieces to a large bowl.
Meanwhile place a large pot with water on high heat and bring to the boil. Add a pinch of salt and cavatappi pasta to the boiling water and cook for ¾ of the ‘al dente’ cooking time – refer to the cooking time specified in pasta packet instructions. Drain the pasta, then add to the bowl with blended pumpkin. Stir in the Besciamella sauce. Toss to combine.
Ladle 1/3 of the pumpkin pasta into the pumpkin, using the back of a ladle or spoon to press it down and level it. Sprinkle 1/3 of the smoked mozzarella cheese and 1/3 of the Parmigiano Reggiano on top of the pumpkin pasta layer. Continue layering using the remaining pumpkin pasta and smoked mozzarella cheese. Finish with a layer of smoked mozzarella cheese and remaining Parmigiano Reggiano sprinkled to top the 3rd layer of pumpkin pasta. Add knobs of butter (or margarine) on top.
Return the whole pumpkin to the oven (including the pumpkin top in foil) to the side. Bake for a further 30 minutes.
Remove from the oven and allow to rest for a minimum of 10 minutes before serving. To serve the Pasta al forno (pasta bake) of pumpkin, sage and smoked mozzarella, either use a ladle to scoop out the pasta bake or cut the pumpkin into 6 thick wedges and serve – buon appetito.