West Country Cider Mussels - River Cottage

Ingredients

• 50g unsalted butter
• 1 onion, peeled and finely sliced
• 2 garlic cloves, peeled and finely sliced
• 1 tsp thyme leaves, or 2 tsp wild thyme (if you can find any!)
• 1½ tsp cider vinegar (optional)
• ½ glass of medium cider
• 1kg mussels, purged,scrubbed and debearded
• Sea salt and freshly ground black pepper
• 2 tbsp double cream (optional)