Sprinkle the lemon juice and salt over the meat. Mix the onion, garlic and chili together well. Heat the ghi in a heavy saucepan over medium heat and fry the onion mixture for 2 minutes. Add the coriander, cumin, turmeric, ginger and pepper, stir well and cook another 2-3 minutes. Add the lamb and lemon juice; stir well to coat the meat with the spices. Cook 5-10 minutes. Stir in the tomato paste and beef stock, bring to a boil then cover and simmer gently 30-40 minutes until the meat begins to get tender. Sprinkle in the garam masala and cook for a further 10 minutes. The gravy should by this stage be very thick; if it is not, remove the lid from the pan and increase the heat to boil off excess moisture until the gravy thickens.