Spiced roast lamb

INGREDIENTS

• 2kg leg of lamb
• 150g natural whole-milk yogurt
• 1 thumb-sized piece ginger , finely grated
• 3 large garlic cloves , crushed
• 1 tbsp tomato purée
• juice ½ lime
• 1 tsp ground cumin
• 1 tsp turmeric
• 1 tsp crushed chilli flakes
• 1 tsp fennel seeds , lightly crushed
• handful coriander , finely chopped