Heat the broth in a medium saucepan and keep it warm on low heat throughout the cooking process.
In a large, heavy-bottomed pot, melt 2 tablespoons of butter over medium heat. Add the diced onion and cook until softened, about 5 minutes.
Stir in the garlic and cook for another minute until fragrant, then add the Arborio rice. Toast the rice, stirring constantly, for about 2 minutes until the edges become translucent.
Pour in the wine and stir until it's completely absorbed by the rice.
Begin adding the warm broth one ladle at a time, stirring frequently. Wait until each addition is almost completely absorbed before adding the next ladle. This process takes about 20-25 minutes.
When the rice is creamy and tender with just a slight bite, stir in the pumpkin puree, remaining butter, Parmesan cheese, sage, and nutmeg.
Season with salt and pepper to taste, then remove from heat.
Serve immediately, garnished with fresh sage leaves and toasted pumpkin seeds if desired. Enjoy while it's warm and creamy!