Prepare the marinated fish. Put the dill, salt, sugar, pepper, and brandy in a large bowl. Mix everything well. Place the salmon in this marinade and dip well in it on all sides. In the end, fold the fillets in half, with the skin facing out. Wrap the fish in foil.
Put the fillets in a bowl, and put a load on top (for example, a jar of water). Refrigerate bowl for 12 hours. After this time, take the fish out of the refrigerator without removing the foil, turn the piece over to the other side, but the load on top, and put it back in the refrigerator for 12 hours. After 12 hours, remove the fish from the marinade and dry with a paper towel. Cut the fillet into small thin pieces.
Preheat the oven to 200 degrees. Grease an ovenproof dish with butter.
Put half the potatoes in the prepared dish, on it a layer of fish and onions. Sprinkle everything with dill and top with the remaining potatoes.
In a bowl, combine eggs, milk, salt, and pepper, whisk lightly with a whisk or fork. Pour over the potato casserole with this mixture, top with the remaining butter, and bake for about 1 hour, until the potatoes are soft.
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