For the sausages, heat the oil in a frying pan over a medium heat and fry the chipolatas for 6-7 minutes, or until cooked through and golden-brown on all sides.
Add the red onion slices to the same frying pan and fry for two minutes with the sausages. Add the stock cube half, a splash of boiling water and the soy sauce and cook over a medium heat for 2-3 minutes until the liquid has reduced.
For the mash, reheat the fresh mash in a pan and stir in the wholegrain mustard.
To serve, spoon the mustard mash onto a serving plate, arrange the sausages to one side, and pour the onion gravy over the sausages.