Preheat the oven to 400 degrees Fahrenheit.
Butter a 9 x 13 glass baking dish. Spread salmon, dill, capers, and onion in the bottom of the dish.
Toss potatoes with lemon zest and lemon juice and season with salt and pepper.
In a small bowl whisk together creme fraiche or sour cream and cream, stir into the potato mixture, and over salmon with potatoes.
Sprinkle with cornflake or bread crumbs and scatter small pieces of butter on top.
Cover with aluminum foil and bake for 25 minutes; remove foil and bake uncovered until potatoes are tender and top is browned, about 20 minutes more.