Shell beans and boil them in enough water to cover for 5 minutes. Drain. Cook onions in olive oil until transparent. Add tomato and cook 5 minutes longer. Stir in beans, pepper, salt, sugar and 1 1/2 cups water. Cover and simmer for 1 hour. Add a handful of parsley, cook for a few minutes. Remove from heat. Serve hot, sprinkled with lemon juice or allow to cool and serve cold with lemon juice.