Take the pike perch fillet out of the refrigerator fifteen minutes before cooking and pat the skin bone-dry. Sprinkle it with salt and let the fillet sit; the salt will draw out more moisture, which you should pat dry again just before cooking.
Slice the leeks into rings and rinse them thoroughly in a colander. Melt half of the butter in a pan, add the leeks with a pinch of salt, cover, and braise over low heat for fifteen minutes until soft and sweet.
Sauté the finely chopped shallot in a second pan in a knob of butter, deglaze with the white wine, and reduce by half. Pour in the cream and let the sauce simmer gently for five minutes to thicken. Stir in the mustard only after removing from the heat, as boiling makes mustard bitter and destroys its sharpness.
Dry-toast the flaked almonds in a frying pan until lightly golden and boil the haricots verts for five minutes until tender-crisp.
Heat the olive oil in a frying pan until very hot and place the fillet skin-side down in the pan. Press it flat with a spatula for the first few seconds, otherwise the skin will shrink and the fish will curl up. Fry for four minutes until the skin is crispy, flip, and remove the pan from the heat after one minute.
Serve the fish on top of the leeks with the sauce, the green beans, the flaked almonds, dill, and lemon.