Melt the butter in a large, non-stick frying pan
Add the leek and cabbage and fry on a medium heat for 5-7 minutes, stirring regularly until soft.
Stir in the flour on the heat for a minute until bubbling.
Add half the stock and stir until it comes to the boil. Add the remaining stock and do the same.
Stir in the cream, mustard and season generously with salt and pepper.
Gently bring it to the boil until it's just bubbling, before serving.
