Start by frying the onions, garlic, and celery in some hot oil. After 3 minutes you can add the mushrooms. Season and cook for another 5 minutes.
Now add the rice and the pumpkin puree. Start by gradually adding stock to the pan, one ladle at a time. This assures that the rice gives in all of its nice starch, which makes the sauce thicker.
When all the stock has gone in, the rice should be almost completely soft, but still with a little bite into it. Now add the butter and take off the heat.It should still have some sauce left, which makes your risotto creamy and delicious! Enjoy!