Heat butter in a large non-stick frying pan set over medium heat. Add salmon and cook until underside is light brown, about 2 minute Turn salmon and continue cooking until browned 2 more minutes. Reduce heat to low, cover continue to cook for 6 more minutes. Remove to plate and cover to keep warm.
Meanwhile, slice zucchini lengthwise into four strips, then thinly slice and add to pan. Stir constantly over medium high heat until slightly softened, about one minute. Stir in broth. Bring to a boil. Cook, uncovered and stirring often until reduced by half, about 2 minutes.
Stir in sour cream, mustard and dill weed. Gently stir for 2 minutes. Finely slice onions, and stir into sauce.
Add salmon and cook until just heated through, about one minute. Season with salt and pepper.