Mix all the marinating ingredients in a bowl and add the mutton pieces to it. Mix well and let it marinate in the refrigerator for at least 12 hours. Do not add salt to the marinade.
When the mutton has marinated for long enough, take a pressure cooker and heat the remaining mustard oil in it.
Temper the oil with the whole spices (about 30 seconds) and add the onion paste.
When the onion turns slightly brown add the ginger and garlic paste.
Saute over medium heat for 4-5 minutes and then add the tomato puree. Cook until the tomato has turned a darker red.
Add the cumin powder, chili powder, and coriander powder and cook for 2 more minutes.
Now add potatoes and the mutton pieces with the marinade.
Mix well before adding the water.
When it comes to a boil, cover with the lid and pressure cook until the meat is soft (takes about 20-35 minutes depending on the cooker).
When the steam has escaped, open the lid and let the meat slow cook for at least 90 minutes, stirring ocassionaly. You may want to take out the potatoes to prevent over cooking.
Sprinkle the garam masala powder just before turning off the heat.
Serve with steamed rice or roti.