Preheat the oven to 200°C (390°F).
Begin by roasting the pumpkin. You will only need half, about 500 g (1.1 lb). Cut it open, remove the seeds with a spoon and place the pumpkin on a baking sheet lined with parchment paper. Season the pumpkin with olive oil, salt and pepper, rub the surface of the pumpkin with your fingers to distribute the seasonings.
Roast it for about 45 minutes, until the pumpkin is golden, almost caramelized on the edges, with a pulp so soft that it can be scoop out with a spoon.
Remove it from the oven and set it aside.
While the pumpkin is roasting in the oven, prepare the vegetable stock. Wash the carrot, celery and parsley and peel the onion. Cut the carrot and celery into large chunks, and divide the onion in half.
In a small pot pour the water, and add carrot, celery, parsley and onion. Add the salt, too.
Bring the water to a boil over medium heat. When it starts boiling, lower the heat and let it simmer for about 20 minutes. The water will reduce by half, and it will become a light vegetable stock. Put this aside too. If you use it immediately you will not need to reheat it, but if you make it well in advance, be sure to warm it up before using it for the risotto.
Now it's time for the risotto. Finely chop the onion (if you do not feel to master the knife skills properly, use a cutter, but do not chop it too much or it will become a mush).
Pour the olive oil in a pan, add the chopped onion and half a teaspoon of salt: the salt will help you to cook the onion without burning it. Sauté the onion in the olive oil over low heat until it becomes translucent and soft. Just a few minutes will be enough.
Add the rice and toast it. Stir it with a wooden spoon into the sautéed onion and let it toast, always stirring, over medium-low heat, until the rice will be translucent, almost pearly. If you listen carefully to the pot, your will hear it crackle imperceptibly. Again, a few minutes will be enough.
Pour the white wine over the rice: let it evaporate, stirring with the wooden spoon, until the rice has absorbed it.
Now cook the risotto by pouring the hot vegetable stock in at least 4 times. Cook the risotto over medium-low heat, stirring very often. Doing this, the risotto will become creamier thanks to the starch released by the rice.
Halfway through the cooking, when you have used about half of the vegetable stock, add the pumpkin pulp by spoonfuls. Stir thoroughly and you will see that the pumpkin will melt in an orange cream and will also colour the grains of rice.
Keep on adding the stock to cook the rice. This whole process will take about 10 minutes, not more, so remember to taste the risotto every now and then. It will be ready when you feel it soft, but still with a hard soul inside, slightly al dente.
When you have finished the stock, or maybe you’ll have just a tiny bit left, and when the rice will be cooked but still al dente, remove the risotto from the heat, add the butter and the grated cheese and stir well. This is what we call mantecare, in Italian, which means almost to cream the butter and cheese into the rice. Taste it one last time to see if it still needs a little salt and you are ready to serve it in the dishes.
Sprinkle with black pepper, and add some toasted hazelnut. Enjoy your meal.