Simmer the fish stock
30 minRinse the fish heads and bones under cold water, then place them in a pot with 2.5 liters cold water. Add the green onions, thyme, allspice, crushed garlic, and black pepper. Slowly bring the water just to a simmer and skim off the gray foam that rises to the surface. Reduce the heat and simmer for 30 minutes without letting the water boil.