Preheat the oven to 180°C.
Toss the cubed pumpkin in olive oil, season with salt and pepper and roast in the oven until golden brown and soft in the middle. This will take about 30 minutes. Warm the vegetable stock and leave aside.
Heat a medium sized frying pan while the pumpkin is roasting and heat some olive oil with half of the butter. Add the onion and cook for 3 minutes until softened. Mix in the celery, garlic, herbs and a little pepper. Cover the pan and cook for about 2 minutes.
Turn the heat up slightly and mix in the rice. Stir for a minute, add in the wine and allow to simmer.
Reduce the heat and begin adding the stock gradually. Stir the mixture gently between adding large spoonfuls of stock. This will help the rice absorb the liquid. Wait until the stock has been absorbed before adding another spoon/ladleful. It will take 15-20mins for all the stock to be absorbed.
Remove from the heat; add the rest of the butter and the parmesan to the risotto. Cover for 2 minutes and then stir.
Transfer the risotto to shallow bowls and top with the roasted pumpkin and remaining parmesan.