Combine stock, pumpkin puree, salt, pepper, and nutmeg in a medium pot and bring to a boil. Then reduce to a gentle simmer and keep over low heat.
Melt butter in a medium or large pan over medium heat. Add the onion and cook until slightly transparent, about 5-7 minutes.
Add the rice straight to the pan and stir to coat it with butter. Reduce heat slightly and carefully add the wine. Stir continually until the wine is absorbed.
Once the wine is mostly absorbed, bring the heat back up to medium and begin adding your stock mixture about a ½ cup at a time, stirring constantly and allowing the liquid to be absorbed by the rice before adding more. This should take 30-40 minutes. It should take 5-5½ cups of liquid to reach a creamy but not too soft texture. Taste your risotto to see if it needs more stock or not.
Once risotto is cooked al dente, stir in the grated parmesan cheese and chopped sage leaves. Serve immediately.
