Preheat oven to 220C. Mix one chopped onion, one teaspoon olive oil and two tablespoons balsamic vinegar in a bowl. Spread on a baking tray and put four portobello mushrooms on top (stem side up). Cook for 15 minutes.
Remove and crumble 50g of blue cheese into the mushrooms and cook for a further five minutes. Toast one large ciabatta roll and then half. Place a bed of rocket, onions and two mushrooms in each. Serve with oven chips.