Preheat oven to 200°C. Heat olive oil and butter. When bubbling, add onions, cover and cook gently for 20 minutes, stirring occasionally. Add sugar and vinegar and cook for another 10 minutes. Set aside and allow to cool. Line a baking tray with baking paper. Lay puff pastry out on paper and spread with cooled onions, leaving a 3cm border around edges. Randomly scatter sliced chorizo over onions and sprinkle with sage. Brush edges of pastry with egg and bake for 25 minutes until edges are browned and puffed up. Serve hot.