Preheat the oven to 180°C / 350°F / Gas Mark 4.
Peel and dice the pumpkin in cubes about 1.5cm long.
Put in a bowl with enough oil to lightly coat and toss with salt and pepper to taste. Place the pumpkin on an oven tray with the rosemary and cook for about 30 minutes in the oven.
Put a pot of salted water to boil.
Add a drop of oil and the sliced onions to a pan. Add the cream when golden and allow it to sit over medium heat for 5-10 minutes.
Season with salt and pepper to taste. When ready set aside.
When the pumpkin is ready save a third of the dices and add the rest with the cream sauce. Pour in a blender and mix until smooth.
Cook the pasta for 5 minutes until al dente.
When the pasta is ready mix in with the pumpkin and cream sauce and the black olives.
Plate while still warm and top with the pieces of roasted pumpkin saved earlier with a few leaves of fresh basil.
Finish with parmesan to taste.