Heat the oil in a non-reactive medium saucepan. Add half of the onions and cook over moderately high heat until slightly translucent. Stir in the pumpkin/squash, wine, nutmeg, white pepper, and salt, and cook, stirring occasionally, until most of the liquid has evaporated, about 8 minutes. Remove from the heat and let cool slightly.
In a food processor, purée the pumpkin mixture until smooth. Transfer to a small bowl.
In a medium saucepan, bring the stock to a boil over moderate heat. Reduce the heat to low and keep the stock hot.
In a non-reactive 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 more 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 purée. Continue adding the remaining 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.
During the last 10 minutes of cooking, heat a frying pan and sear the scallops, season with salt and pepper.
Stir in the parsley, leaving some for garnish, the remaining 2 1/2 tablespoons butter, and cheese into the risotto.
Spoon the risotto into 6 warmed soup plates. Place 2 seared scallops on top and sprinkle the remaining cheese and parsley on top. Serve immediately.