Carefully cut off the top of the pumpkin. Set aside.
Scoop out the seeds. Season the inside of the pumpkin with salt and pepper and a drizzle of olive oil.
Place the cleaned pumpkin and the top on a baking tray lined with parchment. Bake until the top is tender. Set the top on a plate to rest and return the rest of the pumpkin to the oven until the flesh is just cooked through, but not too soft. Remove from the oven and let rest until ready to stuff.
Cut in half lengthwise and set on a rimmed baking sheet. Drizzle with olive oil and salt and pepper. Arrange cut side down.
Roast at 400°F for about 45 minutes, or until the flesh is tender enough to shred with a fork. Shred the squash.
Spread on a rimmed baking sheet.
Drizzle with olive oil and season with salt and pepper.
Roast at 400°F for about 15 minutes or until just cooked.
Make the meat sauce
In a large saucepan, saute ground beef in olive oil.
Season with salt.
Stir regularly so the meat is browned evenly. Once the meat begins to caramelize, add the cinnamon, allspice and bay leaf.
Add onions and continue cooking until translucent.
Add wine,tomato puree and tomato paste.
Cook over low heat until thickened.
Season to taste.
Make the bechamel
In Saucepan Cook flour and butter together for a few minutes, until soft.
Quickly whisk in milk and cook over medium heat until smooth and thickened.
Remove from heat and whisk in the cheese.
Whisk in the eggs yolks and nutmeg.
Assemble the dish
Preheat oven to 400 degrees Inside the pumpkin, alternate layers of meat sauce, bechamel, spaghetti squash, celery root and cheese.
Bake for about 30 minutes, or until the pumpkin is heated through. Garnish with toasted pumpkin seeds and fried sage leaves, if using, and serve.