Prepare the filling. Heat the oven to 160°C fan/gas 4. Put the trout on a large sheet of foil, season with salt and pepper, then lay the lemon slices on top and seal the foil to make a parcel. Cook for 15-20 minutes, open the parcel, check the fish is cooked through (opaque and easy to flake), then leave to cool.
Line a 1lb loaf tin with cling film.
Combine the smoked trout with cream cheese, double cream and butter with dill, cornichons and pink peppercorns in a food processor and blitz until smooth.
Line the terrine with cold sliced salmon/ trout allowing some to hang over the edges.
Fill with half the creamy trout mix, sprinkle with dill and pink peppercorns and top with remaining mixture and smooth on top, place one strip of smoked salmon/trout down the middle and then fold over the hanging trimmings.
Cover with cling film and pop a weight on top to compress everything.
Chill in the fridge overnight.
Serve slices of the terrine with pickled cucumber and my delicious Guinness Treacle Brown Bread.