Using the Nasty Bits

• Although restaurant cooking can sometimes be extravagant, at heart, chefs are downright misers who are unwilling to waste anything. That includes the parts of a fish that don't end up on a plate. The bones, skin and heads are filled with flavor compounds, which can be extracted after a minimal amount of simmering. The skeleton usually also contains some of the flesh, left behind when fillets were cut away. The bones and meat provide a clean, fishy flavor, while the head and skin supply gelatin that gives the broth its richness and body.

Trimming Your Tilapia

• If your fishmonger buys tilapia whole and fillets them for market, you can probably arrange to buy as many of the skeletons, or "racks," as you want. Alternatively you can fillet your own at home and save the racks in your freezer until you're ready to make stock. Scale the tilapia before you start, so you can use the skin. Make a vertical cut behind the head, until you reach the spine. Then, turn your knife horizontally and follow the line of the backbone until you reach the tail. Turn over the tilapia and repeat the process on the other side. Cut away the ribs in a single piece, along with the membrane that holds them together.

Taking Stock

• If you have a kitchen scale, weigh your tilapia trimmings. You can use about twice that much water by weight, so if you have two pounds of tilapia, you can make approximately four pints of stock. Cut away the gills from the fish heads--they make the broth bitter--then put the bones in a pot and cover them with the water. Add onions or shallots, celery, parsley stems, a sprig of thyme and some whole peppercorns and pour in a splash of white wine. Bring it all to a boil, reduce the heat and let it simmer for 45 minutes to 1 hour.

Keeping it Cool

• Once your stock is made, it's important to cool it as quickly as possible for food safety reasons. The best way is to strain it into a clean pot or stainless-steel bowl, then set it into a bowl or sink filled with ice water. Stir the stock frequently and replace the ice as needed, until it reaches room temperature. Portion the stock into flat 1- or 2-cup containers and transfer it to the refrigerator or freezer. If you need smaller quantities occasionally for sauce-making, freeze some of the stock in ice-cube trays instead and keep the cubes in a heavy-duty freezer bag.