Thinly slice and sauté the leeks in the butter until soft.
Leave to cool.
In a bowl, grate in the Gruyère and Caerphilly, add in 125g of the breadcrumbs, chopped fresh herbs, mustard, the cooled leeks and egg yolks – stir to combine.
Divide the mixture into 12 and roll into a sausage shape, place on a plate and chill for 20 minutes in the fridge.
Whisk the egg whites until frothy, place the remaining breadcrumbs on a plate and the plain flour on another plate.
Dust the sausages in the flour, then the egg white and finally the breadcrumbs.
Shallow fry until golden brown using the rapeseed oil, then pop into a preheated oven set to 160°C / Fan 140°C / Gas mark 3 for 10 minutes.