In a large, hot, well oiled (or gheed) pan, sear the meat all over, then set aside.
Reduce the heat to medium and top up the oil or ghee, then caramelise the onions for about 10 minutes, until they go very soft and sweet. A little salt will help the process along. After the first five, add the mustard seeds as well.
Now add the spice powder and mix it in, cooking for a couple of minutes.
Add the paste and stir in for about a minute.
Add the chopped tomatoes and stir everything together.
(Optional) Blend everything together and return to the pot.
Add the meat, whole chillies and a bay leaf, then cook on a low-medium heat for 1.5 - 2 hours, until the meat is braised and the sauce is nice and thick. If the meat isn't fully braised by the time the sauce has reduced, top up with a little water. Stir regularly while it cooks to make sure nothing sticks.
Pick out your bay leaf and add the tamarind sauce. As much as you feel the dish needs. If you want to bring even more sour flavours out of the dish, a little lime juice will work very well.
Adjust the seasoning if necessary and serve with rice or naan.