Slice the smoked salmon into thin slices and use the slices to carefully line a terrine mould, leaving the salmon overhanging on each side.
Finely chop all remaining smoked salmon and put into a mixing bowl.
Flake the poached salmon into the same bowl. Beat the butter until very soft, and add to the fish along with the yoghurt and chives. Mix well and season with salt, pepper and lemon juice.
Spoon into the terrine, making sure it is filled to the top. Fold the overhanging salmon in, cover with cling film and refrigerate for several hours until firm.
Slice and serve with toast.