Prepare the fish by removing the scales (you can also buy them already cleaned), cut off the head and tail. Clean the fish bones under cold running water.
Place the fish heads and tails in a large casserole or deep pan along with the fish heads and tails, vegetables and add the bay leaves, peppercorns and salt.
Cover everything with 2 quarts (2 liters) of water and bring to a boil. Then turn the heat down to low and simmer for about 40 minutes until the vegetables have softened. Take the vegetables out from the liquid and reserve.
Wrap the fish steaks in cheesecloth and add it to the water. Simmer it on close to the lowest heat to avoid overcooking and for the fish to break apart in the broth. After 20-25 minutes remove the fish from the pot and leave it aside to cool.
When cool, unwrap the fish and carefully cut into 1 ½ - inch (about 4 cm) thick slices and remove any bones you may find.
Prepare a serving dish (preferably made of glass for the best visual effect) and arrange the fish inside along with thin slices of carrot and, optionally, cooked peas, if you like.
Strain the fish stock to remove any peppercorns or bits of vegetables and soak the contents of 1 gelatin packet in 1/4 cup of cold water.
Add the gelatin to the warm fish stock and heat the liquid back up until almost boiling then turn the heat off. Avoid actually boiling because that will disintegrate the gelatin.
Let the stock cool slightly and then pour it on top of the fish, covering it as much as possible. Add a few slices of sliced carrot, the parsley and any other vegetables you like in the liquid for a pop of color.
When the liquid reaches room temperature, cover the dish and put it in the fridge until set (about 6 hours or overnight).
Once done you can decorate the aspic with more parsley and lemon wedges.
Serve with something acidic on the side like lemon juice or white vinegar.