Slow cooked spiced mutton shoulder with ginger and sweet potato

Ingredients

• 3 tbsp rapeseed oil
• 1 large large onion, finely chopped
• 4 large cloves of garlic cloves - crushed
• 40 g piece of root ginger, grated
• 1 tbsp garam masala
• 1 tsp turmeric
• 1 tsp corriander
• 1/2 tsp ground black pepper
• 2 green chillies, left whole and pricked with the tip of a knife
• 500 g mutton shoulder, diced
• 400 g tin of chopped tomatoes
• 500 ml lamb or chicken stock
• 1 tsp salt
• 1 medium, approx 300g sweet potato, cut into large chunks