Flake salmon, discarding skin and bones.
In bowl, gently combine flaked and smoked salmon, parsley, onions and tarragon.
In separate bowl, beat together butter, mayonnaise, mustard, lemon juice and pepper; add salmon mixture and gently combine.
Line 8 x 4 inch (1.5 L) loaf pan with plastic wrap; spoon in salmon mixture and cover with plastic wrap.
Refrigerate for about 3 hours or until firm. (Terrine can be refrigerated for up to 5 days or wrapped and frozen for up to 1 month.)
Unmould and serve in slices.