Finely chop the onion. Heat a tablespoon of butter in a small saucepan, then steam the onion cubes until translucent. Set aside.
Cut the pumpkin into small cubes.
For a few minutes sear the rice in a larger pot, while continuously stirring.
Add the rosemary sprig to the hot vegetable stock so its aroma can unfold.
Douse the rice with white wine and let all the alcohol evaporate.
Add the chopped pumpkin and browned onions, mix and little by little add the stock.
Stir in the mascarpone 5 minutes before the cooking time ends.
When the rice is cooked, take the pot from the stove and add the grated hard cheese to it. Let the risotto rest a bit, then finish with the hazelnuts and a pinch of dried rosemary on top.