Mix together all of the ingredients for the chicken tikka (apart from the chicken) in a bowl and mix well.
Cut the chicken into a 2cm (¾) dice and add to the marinade, and mix well, cover and allow to sit for at least 4 hours, but overnight is best.
Preheat your grill (broiler in the US) as high as it will go.
Thread the chicken onto skewers.
Cook under the grill for 15 minutes, turning to prevent burning, or until the internal temperature reaches 73°C or 165°F. Set aside and allow to cool.
Prepare the Rice:
Wash the basmati rice thoroughly and allow it to soak in cold water for 20 minutes.
Add all of the remaining ingredients for the rice into a pan (15-16cm or 6") with the cold water and bring to a rolling boil. Then reduce the heat to a gentle simmer.
Add the soaked and washed rice to the water and add a lid.
Cook for 9-10 minutes (the rice should be very nearly cooked), then remove from the heat and allow to steam with the lid on for 5 minutes.
Spread the rice on a plate or tray to cool quickly, and when cooled, keep refrigerated until needed. Pick out the whole spices when cooled.
Prepare the Onions:
Grate the Ginger.
Peel and mash the garlic to a paste.
Slice the green chillies into two pieces lengthways.
Peel the onions, then cut them in half, then into half-moon shapes around 2-3mm (⅛") thick.
Heat the cooking oil in a heavy-based 28cm or 11" frying pan or skillet over a medium-high heat. Add in the curry leaves, slit green chillies, ajwain, mustard, fennel and cumin seeds, then cook for 30 seconds.
Add the garlic and ginger, then cook for another minute.
Throw in the onions and stir, reduce the heat to low-medium and add in the onions. Slowly cook until the onions caramelise; this should take between 45 and 60 minutes.
Deseed the tomato and cut the flesh into a 1cm (½") dice (discard the seeds).
Stir in the turmeric, ground coriander, raisins and tomatoes, stir and set aside until you are ready to build the biryani.
Building your Biryani:
This will fit neatly into a 1-1.2 litre (4-5 cups) baking dish or two separate 500-600ml (2 cups) baking dishes. I prefer the latter.
Add some (a quarter or half, depending on the size of your bowls) onion mix to the base of your bowl, then some chicken tikka, followed by some of the rice.
Gently press everything down and then repeat with the remaining ingredients, finally pressing things down gently again.
Cover with foil and place in the oven for 40 minutes at 180°C or 350°F.
If you are using saffron, heat the milk in a small pan and add the pinch of saffron. Swirl to unlock the colour, then drizzle over the top of the rice.