Melt butter in a pan and fry sliced leeks for 10 minutes until soft.
Mix 100g breadcrumbs, parsley, thyme, and grated cheese in a bowl.
Beat egg yolks with mustard, salt, and pepper in a separate bowl.
Combine the softened leeks, breadcrumb mixture, and egg yolks using a wooden spoon.
Shape the mixture into eight sausages and place on a lined tray.
Whisk egg whites until frothy and spread 40g breadcrumbs on a plate.
Coat sausages in egg whites then breadcrumbs and chill for 30 minutes.
Fry onions in oil for 20 minutes until soft for the relish.
Add chilli and garlic to the onions and cook for five minutes.
Stir in sugar and vinegar, simmering until the liquid reduces and thickens.
Pan-fry sausages in oil for 12 minutes until golden-brown and crisp.
Serve sausages with the cooled relish.