Cheesy gratin of leeks and brussels sprouts

8 medium leeks or 1 ½ lbs plus ½ lbs quartered, blanched and refreshed sprouts

600ml whole milk

a few slices of carrot and onion

3 or 4 peppercorns

sprig of thyme or parsley

175g grated Cheddar cheese or a mixture of grated Cheddar, Parmesan and Gruyère

¼ tsp Dijon mustard

salt

freshly ground black pepper

For the buttered crumbs:

50g butter

110g soft white breadcrumbs