Warm the vegetable broth in a saucepan over low heat until it just simmers; keep it at a gentle simmer throughout cooking.
In a large heavy-bottomed pan, melt the butter over medium heat until it starts to smell nutty and sizzle gently.
Add the finely chopped onion to the pan and cook, stirring often, until translucent and slightly golden, about 5 minutes.
Stir in the Arborio rice, cooking for about 2 minutes until the edges look clear and glassy, releasing a faint aroma.
Pour in the white wine, stirring continuously until it has nearly evaporated and the rice begins to look glossy.
Mix in the pureed pumpkin, stirring well to combine, and cook for another 2 minutes, allowing the flavors to meld.
Begin adding the warm broth, one ladleful at a time, stirring constantly and waiting until each addition is absorbed before adding the next, maintaining a gentle simmer.
Continue stirring and adding broth gradually for about 20-25 minutes, until the rice is tender yet still slightly firm to the bite and the mixture is creamy.
Finely chop the sage leaves and stir them into the risotto, then add the grated Parmesan and a splash of lemon juice, stirring until melted and fragrant.
Taste the risotto and season with salt and pepper as needed, then let it rest for 2 minutes to settle and develop its flavors.
Serve the pumpkin risotto hot, garnished with additional Parmesan and sage leaves if desired, and enjoy its silky, vibrant goodness.
