On each plate (it is important for them to be hot), lay 2 half sole fillets one on top of the other.
Arrange chunks of potato alongside and pour a generous helping of the shellfish and mushroom sauce over.
Garnish with a few leaves of parsley and serve immediately.
Remarks
If you don't have exactly the same shellfish as shown here, use any others that are in season or whatever you have to hand. You can also use prawns or langoustines (scampi).
At stage 10 you will certainly have some of the mussel stock left over. It is too good to throw away, so freeze it in an ice-cube tray, like for beaten egg-yolk. You can then make delicious sauces to accompany fish or shellfish.