Place cod’s roe in a mortar. Trim crusts from bread; soak bread in water; squeeze and add to cod’s roe. Pound mixture to a smooth paste, and stir in onion and garlic. Then add olive oil and lemon juice alternately a little at a time, stirring well, until mixture acquires a smooth, uniform consistency. Strain through a fine sieve. (The above can be done in an electric blender, in which case the mixture does not need to be sieved.)
Serve in a salad bowl, sprinkled with finely chopped parsley and garnished with green olives. Serve with hot toast.