How to Make It
Snap the tough ends off the asparagus and discard them. Cut the spears into 1-inch lengths. In a large pot of boiling, salted water, cook the asparagus until just done, about 5 minutes. Drain. Rinse with cold water and drain thoroughly.
In a large frying pan, heat 2 tablespoons of the oil and 1 tablespoon of the butter over moderately high heat. Add the mushroom caps, 1/2 teaspoon of the salt, and 1/4 teaspoon pepper. Cook, stirring frequently, until browned, about 5 minutes. Add the garlic and thyme and cook, stirring, for 30 seconds longer. Remove from the heat and leave in the pan.
In a medium saucepan, bring the stock to a simmer.
In the large pot, heat the remaining 2 tablespoons oil over moderately low heat. Add the onion and cook, stirring occasionally, until translucent, about 5 minutes. Add the rice and stir until it begins to turn opaque, about 2 minutes.
Add the wine and the remaining 1 1/4 teaspoons salt to the rice and cook, stirring frequently, until all the wine has been absorbed. Add about 1/2 cup of the simmering stock and cook, stirring frequently, until the stock has been absorbed. The rice and stock should bubble gently; adjust the heat as needed. Continue cooking the rice, adding the stock 1/2 cup at a time and allowing the rice to absorb the stock before adding the next 1/2 cup. Cook the rice in this way until tender, 25 to 30 minutes in all. Before adding the last 1/2 cup of stock, stir in the lemon zest. The stock that hasn't been absorbed should be thickened by the starch from the rice. You may not need to use all of the liquid, or you may need more stock or some water.
Stir in the asparagus, half of the mushrooms, the remaining 1 tablespoon butter, the Parmesan, and 1/8 teaspoon pepper and cook, stirring, until the vegetables are heated through. Reheat the remaining mushrooms. Mound the risotto onto plates. Top with the remaining mushrooms. Pass extra Parmesan.