Start by taking the scales off. The scales on flounder are small, so you can remove them easily using a steel wool scrubber like the one pictured. (You can also scrape them off with a kitchen knife.)
Stroke the surface of the fish gently with the stainless steel scrubber. It's easier if you rinse the scales off occasionally with water.
The flounder on the left in this photo has been de-scaled. I could remove them cleanly. (There are scales on the under side too, so be sure to remove them there also.)
So let's start breaking down the fish. There's a white line down the middle of the fish, so cut through the fish along that line, cutting deep enough that the knife hits the back bone.
Make a V-shaped cut along the gills as shown in the photo.
Next, make a cut right along the edge of the front and rear back fins. Your knife will hit the bone right away, so you just need to make a shallow cut.
Do the same on the other side.
Go around the perimeter of the whole fish as shown here. From here on, it's easy. There's just a little more work to do, so hang in there!
Insert your knife alone the center cut. The knife will hit the back bone, so keep cutting into the flesh as you move the knife down.
It looks like this. The flesh will come off the bone gradually. By the way, with this method you won't damage the intestines at all.
If you cut all the way through from the center cut towards the cuts you made around the perimeter, the flesh will come off nicely like this. One quarter of the fish is now filleted.
Cut the other side in the same way to take the other half fillet off. Turn the fish over, and cut the fillets off the other side in the same way.
You'll be able to fillet the fish cleanly like this. The '5-piece cut' is now done and you have 5 fillets plus the backbone with the head attached.
From here, peel the skin off to use the fillets as sashimi, or score the skin if you're using it in a meuniere. Oh! And finally, how to take off the fin muscle.
When you remove the skin, the fin muscle will come off like this easily. Please try it out.
Story Behind this Recipe
I wanted the clean the flounder that I caught myself, so I did it through trial and error. It's the way an amateur like me would do it, but you will get 5 nice pieces out of it.
It's much easier if you remove the scales at the beginning completely. Once you carefully cut around the fish to make the guidelines, the rest is easy. I thought it was difficult to remove the fin muscle, but with this method it comes of really, really easily.