Pour the boiling water over the tamarind pulp and allow it to sit for 10 minutes, then mash it and pass it through a fine mesh sieve.
Mix together the turmeric, amchur, cumin, Kashmiri chilli powder and half of the salt.
Take the duck breasts, trim them of any sinew and cut a 1cm (½") crosshatch or diamond pattern on the skin, then rub them with the spice mix.
Place the duck breasts, skin side down, in a cold 30cm or 12" skillet or frying pan and then turn on the heat to a little below medium and allow to sit for 8-10 minutes. Do not be tempted to look for at least 6 minutes! The duck should slowly begin to leech fat and then begin to sizzle.
Peel the ginger and grate it.
Peel and mash the garlic cloves.
Cut the green chillies in half lengthways.
By now, the duck should be nice and crispy. Remove it, place it on a chopping board and cover with foil.
Drain all but a scant covering of oil from the pan and return it to a medium heat.
Add the curry leaves, slit chillies, fennel seeds and cardamom pod and cook for 60 seconds.
Add the garlic and ginger and cook for another minute.
Pour in the tomato passata and water, then add the tamarind, remaining salt, ground coriander, fenugreek leaves, jaggery and any remaining dry rub from the duck breasts.
Stir and cook for 2 minutes or until it starts to bubble.
Add the duck breasts, skin side up, then reduce the heat to low to medium and cook until the duck reaches your desired level of doneness, which could be anywhere from 5-10 minutes. I aim for 53-54°C or 127°F for a nice rare duck breast.
Remove the duck and rest under foil for 5 minutes.
Add the garam masala and test the sauce for salt, and continue simmering for the resting time.
Carve the duck and serve it on top of the curry sauce.
Notes
Brushing a naan bread with the set aside duck fat is very, very naughty and you absolutely must do it!