Cook pasta according to package directions until al dente; drain and set aside. Add butter to large skillet and sauté mushrooms over medium high heat until brown; remove from skillet and set aside. In same skillet, add olive oil and sauté spinach and garlic until garlic is fragrant and spinach is wilted; remove from heat and set aside.
To make cheese sauce, add milk and flour to small sauce pan and stir over medium heat until milk slightly thickens. Add salt, pepper and nutmeg; stir in sherry. Remove from heat and stir in cheese until thick and melted. Pour sauce over pasta and toss to coat.
Serve immediately in individual bowls topped with sautéed greens, mushrooms and toasted walnuts.