Cook pasta according to package directions until al dente. Reserve 1/2 cup pasta water. Add onions to pot, then immediately drain remaining water. Return onions and pasta to pot.
Add cream cheese, dill, capers and salmon to pot. Toss ingredients together, adding a little bit of the reserved water at a time to form a thin sauce that coats the pasta. You may not need all of the sauce. Season with salt and pepper.