The first thing to know about filleting a fish is that there are two types of fish: round fish and flat fish. Each of them requires a slightly different technique, but skinning the fillet is the same for both methods.
Second, your task will be a lot easier if you invest in a couple of tools. Look for a filleting knife — a long, slender, pointed knife with a thin, flexible blade. Flexibility is key because you'll need to be able to bend the knife in order to get to all the meat. You'll also want to invest in a pair of fish scissors which are serrated and have detachable blades, and a pair of tweezers to pull out pin bones.
To fillet a fish successfully, there's a certain way to hold your knife. It's important to “choke-up” on the knife — curl your thumb and forefinger just over the hilt of the knife. This will give you more control as you're making precise cuts.
There are a couple of things you need to do before you start filleting either type of fish.
1. Rinse and dry the fish. Make sure you wash the fish to remove any contaminants or remaining scales. We assume you are starting with a scaled and gutted fish, which your fishmonger should gladly do for you. Rinse out the cavity as well, and pat dry.
2. Remove the fins and tail. With a pair of fish scissors, cut off all the fins and the tail; some of them can be sharp and cause serious injury.
3. Keep your knife sharp. Hone your knife. Keep a sharpening steel nearby; you'll want to give the knife a few strokes after every cut.
Round fish generally swim upright and have eyes on either side of the head. Some examples of round fish include branzino, snapper, porgy, and whiting. A round fish will yield two fillets, one from each side of the body.
1. Cut behind the gills. Start by making an incision just behind the gills. Hold the knife at a 45-degree angle to the fish and follow the curve of the bone, all the way around. It may help to press down on the blade of the knife, so that it bends.
2. Cut along the top. At the top of the fish, make a shallow (approximately ¼-inch) cut along the bone from head to tail using the tip of the knife. Keep the knife as close to the bone as possible.
3. Expose the top half of the fillet. Starting at the head, begin to open the fish up with the tip of your knife. Press your forefinger down on the blade and make long sweeping motions with your arm towards the backbone. You'll need to make several sweeps along the length of the fillet. Do not saw. Keep the blade as close to the ribs as possible; ideally, you want to hear the sound of metal scraping against bone. The tip of the blade will contact the backbone running down the center of the fish. Work your way towards the tail on this side of the backbone.
4. Expose the bottom half of the fillet. Once you have opened up the fillet all the way to the backbone, begin at the head again and angle the tip of your knife downward towards the backbone. Position the fish so you can see where you're going with the knife. Make the same sweeping motion towards the tail to complete the fillet, pressing down on your knife and keeping it as close as possible to the ribs. Avoid pulling on the fillet; you'll tear up the flesh.
5. Cut out any extra "pectoral" bones. On this side of the backbone, once you've removed the fillet from the body, there are sometimes extra bones you'll need to cut out; they're on the thinner side of the fillet close to where the pectoral fin used to be. You'll find these on fish like branzino. Approach the fillet at an angle to conserve as much of the meat as possible.
6. Remove pin bones. Lastly, feel all along the fillet for any pin bones; these are bones that are found on some types of fish (such as salmon) that stick straight out from the center of the fillet — you probably had to cut through them to remove the fillet. Pull them straight out with a pair of tweezers.
7. Repeat. Repeat on the other side.