Heat the vegetable broth in a pot until it simmers gently, then reduce the heat to keep it warm throughout cooking.
In a large heavy-bottomed pan, melt the butter over medium heat until it begins to bubble softly and smells fragrant.
Add the finely chopped onion to the pan and cook, stirring often, until it turns translucent and releases a sweet aroma, about 5 minutes.
Stir in the Arborio rice and cook for 2 minutes, watching as the edges become slightly translucent and the rice starts to look glassy.
Pour in the white wine, stirring constantly, until the liquid is mostly absorbed and the rice begins to look glossy.
Add the pumpkin puree to the rice mixture, stirring well to incorporate its vibrant color and sweet flavor.
Begin adding the warm broth one ladleful at a time, stirring continuously and allowing each addition to be mostly absorbed before adding the next, creating a creamy consistency.
Continue this process for about 20 minutes, until the rice is tender but still has a slight bite, and the mixture is thick and creamy.
Remove the pan from heat and stir in the grated Parmesan cheese, lemon juice, and a pinch of nutmeg for added warmth and brightness.
Taste and adjust seasoning with more Parmesan or lemon if desired, then sprinkle with chopped sage or thyme and serve hot.
