Sandy I think that I would always defrost fish before cooking, regardless. It will depend on the size of the fish or portions. I would tend to season and put on a suitable plate a little butter and a splash of dry white wine. Cover well with foil to keep moisture in and check progress after 15/20 mins on gas mk 5. Pour off excess water or liquid and use for any sauce you intend to make or dispose of. Then give a few minutes uncovered. Fish will not take long to defrost. Whilst checking if cooked use a small knife to check the heat has got to the middle of the fish, it will be hot to to the tongue.