For the stuffing, soften the onion and celery in the butter, add the bread, sage and stock, season and mix to a soft paste.
With a thin, sharp knife, cut a pocket into the side of each chicken breast, almost all the way through.
Fill the pockets halfway with stuffing, brush the opening with mustard and press shut.
Brush the outside of the chicken with mustard too and rub in the grill seasoning.
Cook on a hot grill or griddle pan, turning often, for about 20 minutes until both chicken and stuffing are cooked through; rest for 5-7 minutes and serve.