Cook noodles according to package directions. Drain and set aside.
Saute onion in hot olive oil in a large saucepan over medium-high heat until tender. Add noodles, walnuts, spinach, and mushrooms; stir well.
Combine cheese and remaining ingredients in a medium bowl; stir well. Add cheese mixture to noodle mixture, stirring well. Spoon into a lightly greased 2 1/2-quart casserole. Cover and bake at 350° for 30 minutes or until thoroughly heated.