Use a saucepan to make a reduction of the shallot and white wine.
Combine the fresh breadcrumbs with the milk and let it sit for 10 minutes. This is called a panade.
Chop the fish fillet coarsely in a food processor and mix with the soft butter, the panade, and the shallot reduction.
Add salt, pepper, a pinch of cayenne chili pepper (optional), potato starch, eggs and blend until smooth and silky.
Transfer the crayfish meat into a mixing bowl and gently combine with the whipping cream, coarsely chopped crayfish tails and parsley.
Grease the terrine tin and preheat the oven to 150°C.Arrange the fish mousse in the terrine tin. To avoid air-pockets, bang the fish terrine on the bench and smooth the top with a tablespoon.
Cover with tin foil and bake the terrine in a water bath (bain-Marie) for 35-40 minutes.
After baking, place the terrine upside down over a wire rack with a catch pan underneath.Unmould after about 10 minutes and use a sharp knife to slice the terrine.
Notes
Choose firm white fish fillet, such as tilapia.
A panade is a mixture of fresh breadcrumbs combined with a liquid such as water, milk or wine. It is incorporated into any ground meat or fish recipe to keep the mixture moist after cooking.