Preheat the oven to 200°C.
Melt the butter in a heavy based saucepan and add the onions, sugar and vinegar. Cook over a medium heat for 15-20 minutes until soft and caramelised. Season to taste.
Spread the onions over the base and top with rosemary, walnuts, courgette and plenty of crumbled Gorgonzola.
Bake in the oven for 15-20 minutes until golden.
Recipe courtesy of Gorgonzola