Soak the lamb bones in plain water with 2 tsp lemon juice for an hour, then drain.
While the bones soak, make the rasam powder. Lightly toast the spices and curry leaves on low heat taking care that the spices do not brown or burn. Cool and grind to a fine powder. Set aside.
Heat 1 tbsp oil in the pressure cooker on medium flame and saute the shallots for 2 minutes. Add the ginger, garlic and saute for a couple of minutes. Then toss in the tomatoes.
Tip in the drained bones and lightly fry for 2-3 minutes. Then add the rasam water and fry for 30 seconds without burning the rasam powder. Add salt and 1 litre of water, close the pressure cooker and cook till it whistles once. Lower the flame and cook for 40 minutes more.
Once the pressure is released, open the cooker and season the rasam with salt if required. Heat the remaining 1 tbsp of oil in a pan. Toss in the mustard seeds and curry leaves and let them crackle.Take them off the flame and add to the rasam along with the fresh coriander.Serve hot in a bowl or with rice.