First off defrost and fillet a whole dover sole. Remove the skin before removing the fillets. Make a small cut at the tail end of the skin. Using a little salt to help your grip pull the skin across the head of the fish taking care not to pull any of the flesh with it. Using a sharp filleting knife remove the four fillets from the fish.
That's the hard part out of the way. Now place the fish bones and skin into a pan of boiling water and simmer for 30 - 40 mins. Strain through fine sieve and season. This stock will be used for the sauce.
Make a veloute as follows: Melt the butter in a saucepan, add the flour to make a roux. Gradually add the fish stock stirring or whisking constantly. Add the milk and correct the seasoning. Simmer for 10 mins on a low heat.
Heat the olive oil in a pan and place the dover sole fillets in for 1 minute each side, remove from pan and keep in the oven on a low heat. Add the shallots and mushrooms to the remaining oil cook for 1 -2 minutes without colouring.
Add the white wine to the pan and turn up the heat. Reduce by half. Gradually add the veloute to the pan stirring constantly. Turn down the heat and finish off with cream and chopped coriander.
Remove the fillets from the oven, arrange on the plates and pour the sauce over the fillets. Serve with long grain rice and asparagus. Finish off with a sparkling of coriander and a squeeze of lemon juice.