Make the breadcrumbs by tearing the bread into pieces and blitzing it in a food processor. Squeeze the excess liquid out of the courgette.
Heat the butter in a frying pan and fry the onion until soft. Add the grated courgette and cook for 3 minutes, until softened.
Mix with the grated cheese, half the breadcrumbs, the egg yolk and seasoning. Shape into 8 sausages about 10 cm (4 in) long, using floured hands. Dip into the lightly beaten egg white and then roll in the remaining breadcrumbs.
Heat some oil in a wok or frying pan and shallow fry the sausages until lightly golden.
