Special lamb biryani

Name: Special lamb biryani
Ingredients: 500g lamb neck or shoulder, chopped into small chunks; 3 tbsp Indian spice mix plus a pinch (see below); 100g yogurt plus extra to serve; ½ lemon juiced; 3 tbsp vegetable oil; 2 large onions halved and sliced; 300g basmati rice; 4 garlic cloves crushed; thumb-sized piece ginger peeled and finely chopped; 4 plum tomatoes chopped or 200g can chopped tomatoes; good pinch saffron; 100g butter; 2 cinnamon sticks; 6 cardamom pods; 4 bay or 6 curry leaves; 2 star anise; 320g sheet ready-rolled puff pastry; 1 egg beaten; 2 tsp nigella seeds; handful chopped coriander or mint; 1 red onion halved and thinly sliced; 2 tbsp coriander seeds; 2 tbsp cumin seeds; 1 tbsp sweet paprika; 2 tsp mild chilli powder (kashmiri chilli powder if you can find it); 2 tsp fenugreek seeds; 3 whole cloves; 1 tsp fennel seeds; 1 tsp ground turmeric; 10 cardamom pods seeds removed and pods discarded