Heat a large pot of salted water to the boil. Add the pasta and cook according to the package directions.
While the water is heating, put a frying pan over medium high heat and add the butter and olive oil. When warm, add the onions and garlic and some salt and pepper, cook until tender, but don't let them take on color. When tender, add the cream and the basil. Let simmer till the pasta is almost done.
Add the salmon a minute or so before the pasta is done. Mix thoroughly.