Preheat the oven to 180 C / Gas 4. Butter a loaf tin.
Cook the salmon in the microwave till it can be just flaked with a fork, about 1 to 2 minutes.
In a blender add cod, eggs, creme fraiche, salt and pepper. Puree till smooth, then tip into a large bowl. Add the salmon and stir gently to combine.
Pour mixture into the prepared loaf tin. Place the loaf tin in a larger baking tin and add boiling water to come up about halfway to the sides of the loaf tin.
Place in the preheated oven, making sure no water spills into the terrine. Bake for 45 minutes. The centre might still be be a bit wobbly but it will set as the terrine cools. Let cool to room temperature and refrigerate for 12 hours before serving.