Heat the vegetable broth in a large saucepan over medium heat until warm and keep it simmering on low heat.
In another pan, heat olive oil and butter over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
Add minced garlic to the onion mixture and cook for another minute, ensuring it does not burn.
Stir in the Arborio rice to the onion and garlic mixture, cooking for 1-2 minutes until the rice is slightly translucent and coated in oil.
Begin adding the warm vegetable broth one ladle at a time, stirring frequently, and only adding more when the liquid is mostly absorbed.
After about 15 minutes, stir in the pumpkin puree and continue adding broth and stirring for another 10 minutes, or until the rice is creamy and al dente.
Remove the pan from heat and stir in the grated Parmesan cheese. Adjust seasoning with salt and black pepper to taste.
Let the risotto sit for a couple of minutes before serving. Garnish with fresh sage leaves if desired.