Constant stirring gives risotto its creamy texture.
Cut squash into eighths; discard seeds. In a large pot filled with 1 inch of simmering water, steam squash on a steaming rack or in a bamboo steamer until tender, 10 to 15 minutes. Scoop flesh from skin and mash lightly.
In a large saucepan, heat stock to a simmer. In a large heavy saucepan over medium heat, melt 1 tablespoon butter. Add oil and shallots; cook for 2 minutes. Add rice; cook, stirring, for 5 minutes.
Add wine to rice, and cook, stirring, until wine is nearly absorbed. Stir in a cup of stock and the squash; cook at a steady simmer until liquid is nearly absorbed. Continue stirring in stock, a ladleful at a time, until rice is creamy and firm but not hard in the center, 15 to 20 minutes. Add nutmeg, salt, and pepper to taste. Add chopped rosemary. Stir in remaining butter and most of the Parmesan. Serve immediately in shallow bowls garnished with the remaining cheese and rosemary sprigs.