Preheat your oven to 200°C / 400°F.
Heat a 30cm / 12 inch cast-iron casserole dish over medium-high heat. Add the olive oil, and then the diced onion.
Cook the onion, stirring occasionally, until it has softened and is beginning to turn golden brown.
Add the garlic, and sauté for another few minutes until the garlic is slightly browned as well.
Add the vermouth to deglaze the pan, stirring to lift up any browned bits that are stuck to the pan, and then remove from the heat.
Add the rice to the dish with the onions.
Stir in pumpkin puree, ginger, cinnamon, nutmeg, salt, and pepper. Stir the mixture to combine well, and then spread it out in an even layer in the casserole dish.
Pour the hot broth over top of the rice mixture, and gently stir. There should be a good layer of liquid over the top of the rice. Transfer carefully to the oven, as your dish may be very full. (Alternatively, I like to put the casserole dish into the oven and then pour the broth over top).
Bake, covered, for 60-75 minutes.
Test the rice and see whether it's cooked through. If needed, add up to 1 cup of extra liquid and bake for an additional 15 minutes (I like to check this at around the 65 minute mark).
When you remove the dish from the oven there may still be a layer of liquid sitting on top. This will continue to absorb as it cools.
Allow the risotto to stand uncovered for 10 minutes, and then, give it a good stir. The rice should be fully cooked, and the risotto quite creamy.
Serve hot with lots of freshly grated Parmesan or nutritional yeast to keep it vegan.