Prepare the mashed potatoes a day ahead. The fish can be prepared very quickly for a fast meal. 

1. Peel the potatoes; cut into large cubes; cook in boiling water with a pinch of salt;
2. drain; mash them by themselves; add a little olive to make the purée smooth, but still firm; season with salt and pepper;
3. place the potatoes onto a sheet of plastic wrap, shaping them and rolling to make a large sausage; refrigerate for 12 hours - you can also just use a baking sheet; the results are not as pretty, but just as good!

Last-minute cooking and presentation

1. Make the dill sauce;
2. preheat your oven to it highest setting; 5 minutes before serving, put the plain cod filets on a baking sheet and place under the broiler;
3. in the meantime, slice the potato terrine and brown the slices on each side in a dry skillet;
4. spoon some of the dill sauce onto each serving plate; place the cod filet on top and top with a slice of the potato terrine; serve immediately.