Clean the fish heads thoroughly under running water. Remove the gills and any other organs inside the head. Cut the heads in half if they are too large.

In a large pot, sauté the chopped onion and minced garlic until the onion is translucent.

Add the chopped carrots and celery to the pot and sauté for a few minutes until they begin to soften.

Add the fish heads to the pot, along with the bay leaf, thyme, parsley, salt, and pepper.

Cover the fish heads with enough water to fully submerge them in the pot.

Bring the water to a boil and then reduce the heat to low. Allow the soup to simmer for 45-60 minutes, or until the fish heads are fully cooked and the soup has a rich, flavorful broth.

Remove the fish heads from the pot and strain the soup to remove any solids.

Serve the soup hot, garnished with fresh herbs or a squeeze of lemon juice.