4 slender leeks (around 575g/1lb 4陆oz total weight), trimmed
1 tsp English or Dijon mustard
20驳/戮辞锄 plain flour
salt and freshly ground black pepper
20驳/戮辞锄 butter, plus extra for greasing
100g/3陆oz mature cheddar cheese, or other cheese, finely grated
200ml/7fl oz milk, semi-skimmed or full-fat
2 tbsp dried white breadcrumbs (ideally panko breadcrumbs), optional