Have the stock simmering on low heat in a saucepan.

Heat the oil or butter over medium heat in a large, heavy nonstick frying pan and add the onion. Cook, stirring, until the onion begins to soften, about 3 minutes, and add the squash, garlic, and about 1/4 teaspoon salt. Cook, stirring, until the squash begins to soften, about 3 minutes, and add the rice. Cook, stirring, until the grains of rice are separate and beginning to crackle.

Stir in the wine and cook over medium heat, stirring constantly. The wine should bubble, but not too quickly. You want some of the flavor to cook into the rice before it evaporates. When the wine has just about evaporated, stir in a ladleful or two of the simmering stock, enough to just cover the rice and squash. The stock should bubble slowly. Cook, stirring often, until it is just about absorbed. Add another ladleful of the stock and continue to cook in this fashion, not too fast and not too slowly, adding more stock when the rice is almost dry, for 20 to 25 minutes. Taste a bit of the rice. Is it cooked through? It should taste chewy but not hard in the middle. Definitely not soft like steamed rice. If it is still hard in the middle, you need to add another ladleful of stock and cook for another 5 minutes or so. Now is the time to ascertain if there is enough salt. Add if necesary.

Add another small ladleful of stock to the rice. Stir in the Parmesan, nutmeg and parsley, and immediately remove from the heat. Add freshly ground pepper, taste one last time and adjust salt. The rice should be creamy. Stir for a couple of seconds, and serve.