Melt butter in large skillet. When butter foams, add onion and saute over medium heat until lightly golden.
Add salmon and saute for a few minutes. Add cream, and a little salt and pepper to taste. Turn off heat, leaving salmon-onion mixture in pan.
Cook pasta until al dente. Drain, then add to the sauteed onion mixture, turning heat to low. Add parsley and Parmesan (or Romano). Toss until pasta and sauce are mixed well.
Serve immediately or sauce will dry out. If it does, just add a bit more cream and mix.