We went to nearby big town yesterday and bought a whole pomfret and a whole Spanish mackerel (cleaned!). I watched several videos about how to fillet the pomfret, assuming it was a "round" fish, rather than "flat." It looked easy enough on the video. Not! When I put my sharp knife along the ribs, which looked like the top of the pomfret and attempted to enter the fish, it did not give at all! Eventually I produced two fillets, but rather clumsy, and I suspect I wasted a good bit of the meat in the doing. What are the tricks?