Clean the fish thoroughly and cut it into large serving pieces.
Cut the onion in half and lightly char the cut sides in a dry skillet until browned.
Fill a large stockpot with water and place over medium heat.
Add the fish, onion, carrots, celery, garlic, and peppercorns.
Bring to a gentle simmer, cover, and cook for about 30 minutes.
Carefully remove the fish from the pot and allow it to cool slightly.
Remove all bones and skin from the fish and set the meat aside.
Add the rice to the simmering broth and cook until tender, about 15 minutes.
Return the deboned fish to the pot.
Stir in the olive oil and season with salt to taste.
Simmer for another 2 to 3 minutes.
Garnish with fresh parsley and serve hot with crusty bread and lemon wedges if desired.