Start by filleting the fish, then squeeze some lemon juice over the fish and sprinkle with a little salt.
Let the fish rest while you make the rest of the dish.
Slice the potatoes and chop the anchovies.
Lay out four double sheets of tin foil.
Divide the potatoes, onions, dill and egg evenly on the four sheets.
Roll the fish filets into small rolls and place on top.
Place one lump of butter on each pack and squeeze some more lemon juice over it.
Wrap the tin foil around the individual servings and place over open fire for about 25 minutes — or until the fish is firm and completely white.