Melt the butter in a frying pan and gently cook the leeks & garlic for 10–12 minutes until soft and sweet. Leave to cool.
In a bowl combine the Caerphilly cheese, cooked leeks, fresh breadcrumbs, herbs, mustard, egg yolks, black pepper and salt. Mix until the mixture holds together when pressed.
Divide into 8 portions and shape into sausage shapes. Chill for 30 minutes.
Roll each sausage in flour, then beaten egg, then breadcrumbs. Chill again for 20 minutes.
Fry in a shallow layer of oil over a medium heat for 8–10 minutes, turning regularly, until deeply golden and crisp.
Alternatively bake at 200°C fan for 20–25 minutes.
