In a bowl, mix the flour with all the other ingredients up to and including the sodium bicarbonate. Add the butter, water and lemon juice, knead into a soft, smooth dough, cover and leave to rise for approx. 2 hrs. at room temperature until doubled in size.
Shape the dough into four balls, place each one on a piece of baking paper, cover and leave to rise for a further 30 mins.
To steam the dough, sit the steaming basket on a wide-bottomed pan. Fill with water until it is just below the bottom of the basket, arrange the dough balls in the steaming basket with the baking paper, cover and cook over a medium heat for approx. 13 mins.
Mix the tomatoes and remaining ingredients in a pan and bring to the boil. Reduce the heat, cover and simmer over a medium heat for approx. 10 mins., remove the lid and simmer for a further 10 mins., leave to cool.
Heat the oil in a wide-bottomed frying pan, brown the chicken in batches for approx. 2 mins. Reduce the heat, add the sauce, simmer for a further 2 mins.
Toast the papaya seeds over a medium heat. Cut open the rolls, place the chicken on the bottom half of each roll and arrange the papaya, papaya seeds, celery and coriander on top, cover with the top half of the bread roll.