Wash the cowpeas and soak them overnight in plenty of water. Pour off the water and bring the beans to the boil in plenty of water along with the bay leaf, skim off the foam, cover and simmer for about 40 minutes. Put the beans in a sieve and let cool a little. Mix in a bowl with the vinegar, olive oil, salt, pepper and sugar and let rest for an hour. Add the onion rings, tomato pieces, chopped mint and chopped parsley. Season to taste with pepper and salt. Place on a platter and garnish with the un chopped mint and parsley. Serve with fresh white bread.