Now make a roux (flour and butter cooked together in a saucepan for a couple of minutes) and add the milk. Once thickened, add the vegetables and leave to cool.
The mix should be pretty thick, but add extra flour if needed. To form the croquettes divide into about 10 balls and dip each of them first into the beaten egg and then into the breadcrumbs. Chill in the fridge for half an hour and then fry till golden brown.