Slice onions, place in heated pan with olive oil. Cook on gentle heat until onions are golden brown and caramelised. Stir in a splash of balsamic vinegar.
Line a flan tray with pastry. Spread caramelised onions over pastry base. Cut chorizo into slices and arrange over top of onions, pressing down firmly.
Beat eggs with milk and pour over flan, making sure to cover the chorizo and onion. Top with parmesan cheese and bake in 180C oven for about 30-35 mins or until golden brown on top and cooked through.