Wash 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 & 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 quart 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 mins or until the heads are cooked through.
Using a small sieve, lined with a wet towel or cheesecloth, pour the liquid into a 2 quart jar. When most is poured off, press lightly on the remains & pour off the rest.
Place the sections of fish in about 2 cups of stock. Bring to a gentle boil & simmer for about 10 minutes or until the fish turns white.
Do NOT overcook or the pieces will fall apart. Use a slotted spoon to remove fish pieces & place on a platter to cool.
When cooled, remove the skin & bones trying to keep the pieces large & not ragged. Cover & cool. Strain the cooking liquid again & add to the rest of the stock. A more intense flavor, stock may be reheated & reduced by a 1/3.
Dissolve the gelatin in 1 cup of fish tock & add. Taste then add salt if needed. Cool & pour in enough stock to cover the bottom of a 6 cup fish mold or glass pan by 1/2"".
Refrigerate until set. Remove the peel from the cooked carrots & 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.
Serve: Run a sharp knife around the edge of the mold. Place a platter on top of the mold and invert. Using a hot damp cloth helps release it from the mold. Garnish with parsley sprigs & lemon slices just before serving.