Cook the potatoes in boiling salted water until very tender, then drain really well. While the potatoes are cooking, fry the onion in a knob of butter until really soft.
Whisk the eggs with some seasoning then tip in the potatoes, onion, salmon and dill, and stir. Heat another knob of butter in a medium, non-stick frying pan.
Pour in the egg mix and leave for a few minutes until the bottom has set. Slide under a grill to set the top then cut into wedges to serve.