Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Melt butter in a large skillet over medium heat. Saute onion in butter until tender.
Stir flour and garlic powder into the butter and onions. Gradually stir in milk. Heat to just below boiling point, and then gradually stir in cheese until the sauce is smooth. Stir in peas and mushrooms., and cook over low heat for 4 minutes.
Toss in smoked salmon, and cook for 2 more minutes. Serve over pasta.