Preheat the oven to 160°C.
Peel and dice onions then peel and grate garlic and ginger. Fry the onions in a glug of oil for a few minutes before adding the garlic and ginger. Spoon in all the spices with the lamb and fry until the meat is coloured.
Dump in the tomatoes, yoghurt and a splash of the chicken stock then cover with a lid or tin foil and bake in the oven for an hour until the lamb becomes just tender.
Pour in the rest of the hot chicken stock and rice, stir, cover again and bake for another 20 minutes. Take out of the oven, stir in the spinach and leave covered for 10 minutes on the side to continue cooking from the residual heat.
Blitz the mint, yoghurt and lemon juice up in a blender with a pinch of salt. Chop up the coriander and slice the chillies.
Serve the biryani straight to the table in the dish with a pile of chopped coriander, chillies and yoghurt for people to help themselves. Serves 4