Heat 1 tablespoon of butter in a medium sauté pan over medium heat. Add mushrooms, asparagus, and salt and saute until asparagus is crisp, tender, and liquid from mushrooms has dissipated, 7 to 10 minutes. Set aside.
Heat chicken stock in a medium saucepan and hold over low heat.
Heat remaining 2 tablespoons butter in large heavy-bottom saucepan over medium-low heat. Add the onions and sweat until translucent, about 5 minutes. Add rice and stir with a wooden spoon until grains become opaque, 2 to 3 minutes. Add wine and stir until wine is absorbed by rice.
Add enough chicken stock just to cover the rice. Stir constantly until liquid is completely absorbed into rice. Repeat 2 to 3 times until all of the stock is absobed, 30 to 35 minutes.
After adding last addition of stock and most of it is absorbed, fold in the mushrooms and asparagus, and stir until risotto is creamy and asparagus is heated through. Remove from heat and stir in nutmeg, lemon zest, Parmesan, and salt and pepper, to taste. Serve immediately.