Heat the oil in a medium saucepan. Add the squash/pumpkin and half of the onions and cook over moderately high heat, stirring frequently, until the pumpkin is just tender, about 7 minutes. Stir in 3/4 cup stock, lemon juice, nutmeg, white pepper, and salt. Cook, stirring occasionally, until most of the liquid has evaporated, about 12 minutes. Remove from the heat and let cool slightly.
In a food processor, puree the pumpkin mixture until smooth. Transfer to a small bowl.
In a medium saucepan, bring the remaining 7 cups of vegetable stock to a boil over moderate heat. Reduce the heat to low and keep the stock hot.
In a medium saucepan, heat 2 1/2 tablespoons of the butter until it begins to sizzle. Add the rice and the remaining onions and cook over moderately high heat, stirring with a wooden spoon, until the onions are translucent, about 7 minutes. Immediately stir in 1 cup of the hot stock and cook, stirring constantly, until all of the liquid has been absorbed, about 2 minutes.
Reduce the heat to moderate and gradually add 3 more cups of the hot stock, 1 cup at a time, stirring and cooking until each cup is almost absorbed before adding the next, about 15 minutes. Stir in the pumpkin puree. Continue adding the remaining 3 cups stock, 1 cup at a time, stirring and cooking as above, until the rice is tender, about 10 minutes longer. The risotto will be quite loose. Stir in the parsley, parmesan, and the remaining 2 1/2 tablespoons butter.
Spoon the risotto into warmed bowls and sprinkle the Parmesan on top. Serve immediately.