The basic logic of filleting a fish without leaving bones is to hold a thin, flexible knife parallel to the spine and run it from head to tail in one motion. When the blade glides along the spine, the flesh separates from the bone with almost nothing left behind.
The fillet usually isn't entirely bone-free after this step; thin bones embedded in the flesh but not attached to the spine, called pin bones, still need to be pulled out one by one with tweezers.
What knife and grip should you use?
The ideal knife for filleting is a thin, long, flexible fillet knife; that flexibility lets the blade follow the line of the bones. A regular kitchen knife can fillet a fish too, but the thicker the blade, the more flesh it tends to take off the bone with it.
The fish should sit on a dry surface, not a wet one, on a sturdy cutting board to keep it from slipping; gripping the fish firmly by the tail lets the other hand guide the knife with control.
How do you cut along the spine?
Starting just behind the gills, the knife runs right along the spine, following the line where the flesh meets the bone, toward the tail. Keeping the blade angle steady lets the flesh come away from the spine in one piece, leaving almost nothing on the bone.
Near the rib bones, the knife is lifted slightly to pass just above them; taking this part slowly is what keeps the fillet in one whole piece.
How do you remove pin bones?
After the fillet comes off, running fingertips gently from head to tail over the flesh reveals small, firm tips — these are thin bones embedded in the flesh, not attached to the spine. Gripping each one firmly with tweezers and pulling gently along the direction of the flesh's grain removes it.
Fish like salmon and trout have quite noticeable pin bones; fish like sea bream and sea bass have fewer, but not none, so this check should be done on every fish.
How do you skin it cleanly?
To skin a fillet, it's flipped skin-side down, a small cut is made at the tail end, and the knife is angled slightly between skin and flesh, held steady against the skin while pushed forward. Too steep an angle tears the flesh; too flat leaves flesh stuck to the skin.
Pulling the skin back while keeping the knife steady, rather than moving the knife, gives more control; this technique works especially well on thin-skinned fish. Holding the edge of the skin with a damp cloth or paper towel in the other hand stops it from slipping and makes the whole motion easier.
Does the technique change for small fish like anchovies?
For small fish like anchovies, filleting is usually done by hand instead of with a separate cut along the spine: once the belly is opened and the innards removed, the spine is held near the head and pulled toward the tail in one motion. This usually lifts the spine out in one piece, bones attached.
This doesn't work for larger fish, since their bone structure is too firmly attached to the flesh to separate by hand; for those, cutting along the spine with a knife is the only realistic method.