Roast squash and sweet potato soup with buttermilk blue cheese sauce

INGREDIENTS

• 1 onion
• 1 butternut squash
• 500g/1lb 2oz sweet potatoes, unpeeled, cut into 2.5cm/1in thick rings
• 60ml/2fl oz olive oil
• 1 tsp ground cinnamon
• ½ tsp ground nutmeg
• 1.5 litres/2 pints 13fl oz vegetable stock
• 125ml/4oz marsala wine
• salt and freshly ground black pepper
• 125g/4oz blue cheese
• 250ml/9fl oz buttermilk