Cook pasta in a large pot of boiling salted water until al dente, according to package instructions. Reserve 1/2 cup pasta water. Add onion to pot; immediately drain pasta mixture, and return to pot.
Add cream cheese, dill, capers, and salmon to pasta. Toss, adding reserved pasta water a little at a time to create a thin sauce that coats farfalle (you may not need all the water). Season lightly with salt and pepper. Serve.