Fish cakes with mustard sauce

For the fish cakes:

900ml milk

1 bay leaf

300g fish

500g potatoes, mashed

1 zest of a lemon

2tsp Dijon mustard

30g chopped soft herbs (such as parsley, chives, dill)

80g plain flour

1 egg

80ml milk

150g panko breadcrumbs

4tb rapeseed oil

For the mustard sauce:

700ml of milk reserved from poaching fish

35g butter

35g flour

2tb wholegrain mustard

1 lemon juiced