Melt one tablespoon of butter in a large skillet over medium heat, then add your sliced onions, garlic, salt, and pepper. Cook slowly, stirring occasionally, for about 30 minutes until the onions turn deep golden brown. This step builds most of the dish's flavor, so don't rush it.
While the onions cook, pour your vegetable stock into a saucepan and heat it over medium until it's simmering gently. Keeping the stock hot the whole time helps the rice absorb it evenly instead of cooling everything down.
Once your onions are caramelized, add the arborio rice directly into the skillet. Stir constantly for about two minutes so the grains toast slightly and pick up flavor from the pan. Then pour in the white wine to deglaze, scraping up any browned bits from the bottom.
Ladle in about half a cup of warm stock along with the nutmeg, then stir until it's fully absorbed before adding the next half cup. Repeat this process patiently, stirring often, until all the stock is used and the rice turns tender and creamy.
Once your rice is fully cooked, stir in the pumpkin puree and parmesan cheese until everything turns glossy and well combined. Taste it and adjust the salt and pepper as needed before pulling it off the heat.
Spoon the risotto into bowls and finish with your toasted hazelnuts and crispy sage leaves if using. Serve it right away while it's warm and at its creamiest.