Creamy Mushroom Soup - River Cottage

Ingredients

• 30g butter
• 2 leeks, trimmed (white and pale green part only), washed and finely sliced
• A sprig of thyme
• 750g mushrooms, roughly chopped
• 1 small garlic clove, chopped
• 1 tablespoon plain flour
• 1.2 litres hot mushroom or vegetable stock
• 100ml double cream, plus extra to finish
• A few gratings of nutmeg
• 2 tablespoons dry sherry (optional)
• Sea salt and freshly ground black pepper
• A handful of chives, tarragon or parsley, finely chopped,