Cut the leeks into thin pieces. Heat the oil and butter in a frying pan and gently fry the leeks and courgettes for 10 mins until soft. Meanwhile, boil the potatoes until tender and drain.
Mash the potatoes, add the leeks, courgettes and grated parmesan. Mould into small sausage shapes.
Toast the bread and make into breadcrumbs by whizzing in a blender, mix in a bowl with the herbs.
Crack the egg into another bowl and whisk lightly. Put the flour into a separate bowl. Dip the croquettes into the flour, then the egg, then the breadcrumb mixture. Put onto a plate, cover and put into the fridge for 15 minutes.
Preheat the oven to 180?C, fan 160?C, gas 4, brush a little oil on a roasting tray, put the croquettes onto the tray and bake for 10 mins until golden, turn over and cook for another 10 mins.