1. Bring 1 inch of water to boil in large pot. Put asparagus in steamer basket, then carefully place steamer basket in pot. Cover and steam over medium-high heat until asparagus is just tender, about 2 minutes. Remove asparagus and set aside. Reserve 3/4 cup of the asparagus steaming liquid.
2. Heat oil in large skillet over medium heat. Add shallots; sauté, stirring occasionally to separate rings, until crisp and light brown, about 10 minutes. Add mushrooms, increase heat to medium-high, and sauté until they soften and release their juices, about 5 minutes longer. Season with 1 teaspoon salt and 1/2 teaspoon pepper or to taste.
3. Add asparagus and reserved cooking liquid. Simmer until asparagus is heated through and sauce has thickened a bit, about 2 minutes longer. Adjust seasonings. Toss with pasta and serve.