a pestle and mortar or a liquidiser or food-processor
Cut the bread into cubes and soak in the fish soup. Put them in a bowl and work to a paste.
Crush the garlic to a fine purée in the mortar, and add the saffron, the egg yolk and the bread mixture (1). Work the mixture well. Then, using a circular movement of the pestle, incorporate the oil, salt and pepper as if making a mayonnaise. If the mixture turns oily or curdles, add two large tablespoons of hot fish soup and continue to stir.
If you are using a liquidiser or food-processor, follow the same method at the lowest possible speed. Serve in a sauceboat or bowl, and spread on rounds of French bread, which are then floated in the bowls of soup.