Clean the mussels, discarding any that are broken or do not shut tightly when tapped.
Heat the oil in a large casserole and fry the onion gently. When it has softened add the garlic together with the tomatoes and their juice. Cook until reduced to a sauce, then add 5 fl oz water.
Add the mussels to the pan in 2-3 batches and cook each batch, covered, for 3-4 minutes until opened. Use a slotted spoon to remove to a plate and discard the top shells.
When all the mussels are cooked return to the pan and sprinkle with the anis or pernod.
Add about 2/3 pint water and return briefly to a simmer. Season with salt, pepper, cayenne, lemon juice and parsley.
Break a slice of bread up into each of four soup bowls then ladle in the soup and serve.