Pack half of the mixture into the lined terrine. Add the duck breasts and fat strips, then cover with the remaining forcemeat.
Cover the contents of the terrine with foil and a good lid (to weight it down and keep the filling from puffing up and running over the side) and bake in a larger pan filled with water to half the depth of the terrine.
Bake at 280°F for 2 to 3 hours or until the juices are clear. Remove lid and cool.