In a large frying pan, melt the butter with the oil over moderate heat.
Add the mushrooms and salt and cook, stirring occasionally, until the mushrooms are tender and well browned, about 8 minutes.
Add the chicken broth and Boursin cheese and bring to a simmer while stirring.
Snap the tough ends off the asparagus and discard them.
Cut the spears into 1/2-inch pieces. In a large pot of boiling, salted water, cook the pasta until almost done, about 6 minutes.
Add the asparagus and cook until it and the pasta are just done, about 4 minutes longer.
Drain. Toss with the mushrooms and sauce.
Convenient Portobellos: Many stores carry packages of already stemmed and sliced portobello mushrooms, a real convenience. You can use ten ounces of the presliced portobello caps in place of the one pound of whole mushrooms we call for here.
Wine Recommendation: Asparagus is best with a high-acid white wine such as a Bordeaux blanc from France or a California sauvignon blanc. These wines will also balance the rich cheese.