Put half the onion in a large frying pan. Add water and bring to the boil. Add mussels. Cover and cook for 8 minutes or until mussels open.
Remove from heat. Drain, reserving ¾ cup of cooking liquid. Discard any mussels which do not open.
Arrange mussels on a large serving platter. Set aside.
Melt butter in a saucepan. Add remaining onion and garlic. Cook until onion is clear. Stir in flour and cook until frothy. Gradually add tomato purée and reserved cooking liquid. Bring to the boil. Boil for 1 minute. Season with pepper to taste. Pour sauce over mussels.
Garnish with chopped parsley. Serve with crusty French bread.