Bring a large pan of lightly salted water to the boil. Add pasta and cook according to the pack instructions or until al dente, then drain.
Melt butter in another large pan. Saute onion in butter until tender.
Stir flour and garlic powder into the butter and onions. Gradually add the milk, stirring continuously to prevent lumps. Slowly bring to the boil, and then gradually stir in cheese until the sauce is smooth. Stir in peas and mushrooms and cook over low heat for 3 to 4 minutes.
Add smoked salmon, stir through the pasta and cook for 2 more minutes.
Garnish with cherry tomatoes, snipped chives and parsley (optional).