Preheat oven to 200C. Line a baking tray with non-stick baking paper. Place pumpkin on the tray. Spray with olive oil spray. Season with salt and pepper. Bake for 1 hour or until tender. Set aside for 10 minutes to cool.
Meanwhile, heat a saucepan over medium heat. Spray with olive oil spray. Add the leek. Cook, stirring, for 5 minutes. Add chicken and spinach. Cook, stirring, for 1-2 minutes or until spinach wilts. Transfer to a heatproof bowl.
Melt the butter in the pan until foaming. Add the flour. Cook, stirring, for 30 seconds. Remove from heat. Gradually stir in the milk. Cook, stirring constantly, for 5 minutes until sauce thickens. Stir in the parmesan.
Cook the tortellini in a saucepan of boiling water following packet directions or until al dente. Drain and return to the pan. Add the leek mixture and parmesan mixture to the tortellini. Stir to combine. Stir in the pumpkin.
Transfer to a 2.5L (12-cup) capacity baking dish. Top with cheddar and cook for 30-35 min until golden.