Warm the vegetable broth in a saucepan over low heat until just simmering; keep it hot throughout cooking.
In a large heavy-bottomed pan, heat the olive oil over medium heat. Add the diced onion and cook, stirring occasionally, until translucent and fragrant, about 5 minutes.
Add the Arborio rice to the pan and cook, stirring constantly, until the edges turn translucent and it starts to toast slightly, about 2 minutes.
Stir in the pumpkin puree, allowing it to meld with the rice and onion mixture for about 1 minute, releasing a warm, pumpkin aroma.
Begin adding the hot broth one ladleful at a time, stirring gently after each addition until the liquid is mostly absorbed. Repeat this process, maintaining a gentle simmer, until the rice is creamy and just tender, about 18-20 minutes.
Once the rice reaches the perfect al dente texture, remove the pan from heat and stir in the grated Parmesan cheese and lemon juice, mixing until smooth and creamy.
Let the risotto rest for a couple of minutes to thicken slightly, then taste and adjust seasoning if needed.
Meanwhile, fry the sage leaves in a little oil until crispy and fragrant, about 1-2 minutes, then drain on paper towels.
Serve the pumpkin risotto hot, garnished with the crispy sage leaves and toasted pumpkin seeds for added texture and flavor.