Wast the whole fish and fish heads in cold water. Run a knife over the fish, scraping from tail to head, to remove any leftover scales. Rinse the gut cavity. Cut off the head at the gills, rinse again and cut into equal pieces. Rub inside and out with salt and set aside. Remove the gills from the heads. Combine the fish heads, tail, fins and all ingredients in a 4 litre pot, then cover with 6 cups of cold water. Bring to a boil, skimming off the foam, then reduce to a gentle simmer. Cook for 20 minutes or until the heads are cooked through. Using a small sieve, lined with a wet towel or cheesecloth, pour the liquid into a 2 litre jar. When most is poured off, press lightly on the remains and pour off the rest. Place the sections of fish in about 2 cups of stock. Bring to a gentle boil and simmer for about 10 minutes or until the fish turns white. Do NOT overcook or the pieces will fall apart. With a slotted spoon, remove the fish pieces and place on a platter to cool. When cool enough to handle, remove the skin and bones trying to keep the pieces large and not ragged. Cover and cool. Strain the cooking liquid again and add to the rest of the stock. For a more intense flavor, stock may be reheated and reduced by a third. Dissolve the gelatin in 1 cup of fish tock and add. Taste and add salt if needed. Cool. Pour in enough stock to cover the bottom of a 6 cup fish mold or glass pan by 1 cm. Reftigerate until set. Remove the peel from thecooked carrots and slice them into thin rounds. Arrange the carrot rounds in an attractive pattern on the aspic, add a little more cooled stock, and cool until set. Evenly spread the fish pieces over the aspic, covering with more stock, cooling until set. Continue to add stock until the fish is completely covered. Refrigerate overnight. TO SERVE: Run a sharp knife around the edge of the mold. Place a platter on top of the mold and invert. (A hot damp cloth helps to release it.) Garnish with parsley sprigs and lemon slices just before serving.