To ensure uniform pieces of fish that cook at the same rate, buy a whole center-cut fillet and cut it into 4 pieces. If your knife is not sharp enough to easily cut through the skin, try a serrated knife. It is important to keep the skin on during cooking; remove it afterward if you choose not to serve it. Hold a sharp knife at a slight downward angle to the flesh and cut off the whitish, fatty portion of the belly. Cut the salmon fillet into four pieces of equal size to help ensure that they cook at the same rate. Make four or five shallow slashes along the skin side of each piece of fish, being careful not to cut into the flesh.