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
