Add washed mutton pieces to a bowl. Add ginger paste, minced garlic, turmeric, salt and mix well to coat all the pieces properly. Let them marinate for 30-40 minutes.
Now add the marinated mutton pieces to a pressure cooker along with a little water and cook them until done. Meanwhile, add tomato and onion to a blender and blend well to form a thick paste.
Now heat mustard oil in a pan. Let it reach its smoke point. Now add cumin seeds and let them splutter. Add sugar and let it dissolve completely. Add the tomato-onion paste to the pan along with some salt. Mix well and cook the masala for a few minutes. Now add jeera powder, garam masala and again cook by covering it with a lid until half cooked.
Now add the cooked mutton to the pan and mix well to coat them properly in the masala. Add salt as per taste and water as per need to adjust the consistency of the gravy. Cook for 5-10 minutes or until the gravy thickens a bit.
Once cooked, serve the Jeera Mutton after garnishing it with sliced ginger and green chillies.