Place the kneading accessory in the bowl before adding the flour butter, vinegar and a pinch of salt. Let the food processor run until the dough comes together. Add 1-2 tablespoons cold water if the dough stays too crumbly.
Roll out the dough on a floured surface with a floured rolling pin. Fold the dough 3 times, then fold again and roll out into a thin circle. Then fold the dough 3 times (again!), fold once more and cover with cling film. Put the dough in the fridge for 30 minutes.
Pre-heat the oven to 200 °C. Mix the onion, dill, parsley, and lemon zest and season to taste with salt and pepper. Roll out the dough into a thin rectangle and cut off a piece the size of the mould to make a lid. Cut a small hole in the centre of the lid. Cover the bottom and sides with the rest of the dough. Fill the terrine with alternating layers of trout fillet and mixed herbs.
Fold any dough hanging over the edges inwards and put the lid, which you made earlier, on top. Press together to seal. Put the mould one shelf under the centre of the oven and bake for 40-45 minutes until golden.
Meanwhile soak the gelatine. Heat the fish fond and lemon juice. Squeeze all the water out of the gelatine and, stirring, dissolve the gelatine in the hot liquid. Leave to cool and set slightly (it will become somewhat jelly-like). Take the terrine out of the oven and leave for 15 minutes. Pour the gelatine mixture through the hole in the lid with a funnel, until the terrine is completely filled. Then leave to cool, so the liquid can spread and set.
Put the terrine in the fridge for at least 2 hours. Slice and serve.
1 hour preparation time (+ 30 minutes in the fridge)