Marinate the meat in salt, ginger garlic and green chilli paste as well as the yoghurt. Cover the bowl and let the meat marinate for atleast 2 - 3 hours. The longer you marinate the faster your meat will cook and the better it will taste so I recommend you take your time with this - I prefer to marinate this overnight if time permits.
Now, in a earthern pot (if you have one) or a saucepan, heat the oil and add in the finely chopped onions. Fry the onions till they are translucent and completely soft.
Add in all the spices and mix well for 2 - 3 minutes.
Add in the tomatoes you pureed at home as well as the concentrated puree and mix well. Once you have a thick gravy, add in the mutton pieces, jaggery and salt. Give everything a good stir. Cover your pot so that the meat can start slow cooking on a low-medium flame.
Check in on the meat every 10 - 15 minutes. Covering the pot may have made your gravy slightly watery so for the next 15 minutes, leave the pot slightly open so that water can evaporate.
Once the masala has cooked a bit, it is time to adjust the flavour. If it is not spicy enough add in a little more garam masala. If it is too spicy, add in a little more jaggery - remember though that the vinegar is still to come in and that will usually balance everything out. If you can't point out what's missing, it's usually just a bit of salt.
The whole process of the muttton cooking and the tomato gravy evaporating will take about 35 - 40 minutes. When your meat is completely tender, add in the vinegar and cook for a final 5 minutes. If you feel that the gravy has become too sour for your taste, just let the vinegar cook off.
Add in some finely chopped coriander and give it a final stir. Serve hot with LOTS of salli and fresh roti's on the side.