Shepherd's pie with spiced parsnip mash

INGREDIENTS

• 900g/2lbs large parsnips
• 2 onions
• 50g/2oz butter
• large pinch ground cumin
• 1 tbsp freshly ground coriander seeds
• 1 tbsp garam masala
• about 50g/2oz butter
• salt and freshly ground black pepper
• 2 leeks or onions
• 1 carrot
• 3 tbsp dripping or butter
• 4 Italian brown mushrooms
• 450g/1lb cooked roast lamb
• 1 tbsp flour
• 1 tbsp tomato purée
• 250ml/9 oz hot lamb or chicken stock
• 1 bay leaf
• 1 sprig thyme
• 1 tbsp Worcestershire sauce