Heat the stock in a saucepan, bringing it to a simmer and then keeping it covered on low heat.
Place the olive oil, shallots and garlic in a large frying pan and sweat on medium-low heat. When shallots are soft and translucent, add the mushrooms and increase the heat to medium.
Once the mushrooms are browned stir in the rice coating it thoroughly with the mixture. Add one third of the heated stock along with the pumpkin purée. Stir continuously until the rice has absorbed most of the liquid.
Add another third of the stock and stir. After the liquid has been absorbed add the remaining stock as needed along with the peas, nutmeg, salt and pepper.
Once nearly all the liquid is absorbed test rice to ensure it is cooked al dente. If the rice needs to cook more but you've used all the stock, simply add a bit of hot water from the kettle and continue to cook over medium heat. Once the rice has absorbed the water test again.