In an electric kettle or medium saucepan with a lid, combine chicken broth and white wine and heat just to simmering. Keep warm.
In a large 3 to 4-quart heavy saucepan over medium heat, melt the butter. Add the onions and a pinch of salt and sweat until translucent, about 5 minutes. Add the rice and stir. Cook for 3 to 5 minutes or until the grains are translucent around the edges. Be careful not to allow the grains or the onions to brown.
Reduce the heat to low. Add enough of the wine and chicken stock just to cover the top of the rice. Stir or move the pan often, until the liquid is completely absorbed into rice. Once absorbed, add another amount of liquid just to cover the rice and continue stirring or moving as before. There should be just enough liquid left to repeat 1 more time. It should take approximately 35 to 40 minutes for all of the liquid to be absorbed. After the last addition of liquid has been mostly absorbed, add the mushrooms and asparagus and stir until risotto is creamy and asparagus is heated through. Remove from the heat and stir in the Parmesan, lemon zest, and nutmeg. Taste and season, to taste, with salt and freshly ground black pepper.