Start by preheating your oven to 375°F (190°C). Prepare the bell peppers by slicing off the tops and removing the seeds and membranes with a spoon. I've found that a grapefruit spoon, with its serrated edges, works wonders for this task. Arrange the peppers in a baking dish, cut-side up, and lightly brush the insides with olive oil. This not only adds flavor but also helps the peppers soften evenly while roasting.

For the risotto, heat the olive oil in a large, heavy-bottomed pan over medium heat. Add the diced onion and cook until translucent, about 3-4 minutes. I learned early on that rushing this step can lead to a bitter taste, so I always give the onions the time they need to sweeten gently.

Add the minced garlic and chopped sage, cooking for another minute until fragrant. It's crucial not to let the garlic brown, as it can become bitter and overpower the delicate flavors of the sage.

Stir in the Arborio rice, toasting it for about 2 minutes until the edges become slightly translucent. This toasting is key to unlocking the rice's nutty flavor and ensuring that it absorbs the broth properly.

De-glaze the pan with the white wine, scraping up any browned bits from the bottom. Allow the wine to reduce until the pan is nearly dry. This step not only adds complexity to the flavor but also imparts a slight acidity that balances the richness of the risotto.

Begin adding the warm vegetable broth one ladle at a time, stirring constantly and allowing the rice to absorb the liquid before adding more. This gradual addition is the heart of risotto-making; it encourages the rice to release its starches, creating that signature creamy texture. In my early attempts, I added the broth too quickly and ended up with a risotto that was more soup than a sublime creamy dish.

Once the rice is al dente and the risotto has a creamy consistency, stir in the pumpkin puree, butter, grated Parmesan, a pinch of nutmeg, and season with salt and pepper. The pumpkin not only adds a gorgeous color but also a subtle sweetness that complements the sage beautifully. The nutmeg, though used sparingly, is like a secret handshake among spices, enhancing the overall warmth of the dish.

Spoon the risotto into the prepared bell peppers, filling them to the top. I've learned to pack the risotto in snugly, as it will settle slightly during baking. Sprinkle additional Parmesan cheese over each stuffed pepper for a golden, cheesy crust.

Bake in the preheated oven for 25-30 minutes, or until the peppers are tender and the tops are golden brown. The baking time can vary depending on the size of the peppers, so I always check for doneness by piercing the side of a pepper with a fork. It should slide in easily with just a bit of resistance.

Let the stuffed peppers rest for 5 minutes before serving. This brief respite allows the flavors to settle and the cheese to reach its optimal gooeyness. Garnish with fresh sage leaves for an aromatic touch and a nod to the dish's key herb.