Preheat the oven to 220ºC/200ºC Fan/Gas Mark 7. In a large bowl mix together all the ingredients including grated courgette for the burgers. Once evenly mixed, shape into burgers, about 45g each, making about 18.
Pour enough sunflower oil into a large frying pan so you get a thin layer at the bottom, about 2mm thick. Heat up well and sear the meatballs in batches on medium heat from all sides. Cook them for about 4 minutes, adding oil as needed, until golden brown.
Carefully transfer the seared burgers into an oven tray lined with greaseproof paper and place in the oven 5-7 minutes, or until just cooked through. Serve warm or at room temperature, with the sauce on the side.