Cook pasta according to packet instructions.
While pasta is cooking, fry onion in pan, then add the capers and garlic. Be ready with your lemon juice and sour cream.
Drain pasta, reserving 1 cup of cooking liquid. Reduce heat of pan. Put drained pasta into pan with the onions and garlic and half the reserved liquid.
Moving quickly, add the sour cream, stirring so it doesn't separate, and tossing through the pasta gently. Add lemon juice and salmon.
Use the remaining cooking liquid to thin the sauce if it needs to be creamier.
Season with salt and pepper and serve with dill. (I don't hate a little parmesan either but that's up to you.)