Grease 20x30 cm lamington pan (loaftin),cover base and sides with baking paper. In a separate pan bring milk,stock and water to boil.
Gradually add polenta,cook,stirring about 10 mins or until polenta is soft and thick,cool for 1 minute.Stir in cheese and herbs.Spread mixture evenly into prepared pan cover and refrigerate for 2 hours or until firm.
Turn polenta out,cut into 8 pieces.Place polenta on oven tray which has been coated with oven oil spray. Combine extra water and butter in pan,add onions, cook.stirring until onions are soft and water has evaporated.Top polenta with onion micture and extra cheese. Bake in moderate oven about 15 minutes or until cheese has melted.