In a 12 to 14-inch skillet, heat the olive oil over medium heat.
Add the onion and squash cooking until the onions have softened and are
translucent but not browned 8 to 10 minutes.
Add the rice and stir with a wooden spoon until toasted and opaque, 3 to 4
minutes.
Add the wine to the toasting rice, and then add a 4 to 6-ounce ladle of stock
and cook, stirring, until it is absorbed.
Continue adding the stock a ladle at a time, waiting until the liquid is
absorbed before adding more.
Cook until the rice is tender and creamy and yet still a little al dente, about
15 minutes.
Stir in the butter and cheese until well mixed.
Add mascarpone and lemon juice
Portion risotto into 4 warmed serving plates, serving with extra cheese.