Make sure all bones and skin are off the fish, put it into a bowl with the milk and cover with cling film. Microwave at full power for 2–3 minutes or until the fish is opaque. Allow to cool then chill. Do this up to 3 days ahead and store in the fridge.
Gently mix the chives and mascarpone – don’t over-mix because you want the mascarpone to stay stiff. Up to 3