Place the cheese, breadcrumbs, onion, mustard and herbs in a food processor and process until just combined. Season well, then add 1 whole egg and 1 egg yolk (reserving the eggwhite) and process briefly. Use your hands to form mixture into 8 sausage shapes. Whisk eggwhite until just frothy. Roll sausages in eggwhite, then in flour. Heat oil in a frypan over medium heat and cook sausages until brown all over.
To make the sun-dried tomato gravy, heat the oil in a saucepan over low heat, then cook the onion until softened. Add the sun-dried tomato paste and vegetable stock, bring to the boil, then reduce heat to low and simmer for 15 minutes. Season with salt and pepper.
Serve the sausages with the gravy or an onion marmalade (available from gourmet food stores), and mashed potatoes, if desired.
