Grind the cardamom pods into a fine powder. Heat the oil in a pressure cooker and add the cardamom powder and stir. Add the lamb cubes and stir-fry for couple of minutes. Add the onions and tomatoes and stir for another 3 minutes. Then add the tomato paste, garam masala powder and salt. Pour 2 cups of water. Close the lid and let it pressure cook for 15 minutes over medium heat. Reduce the pressure quickly by keeping the pressure cooker under the running cold water with lid on. Uncover the lid and let it cook over high heat with the lid opened. Stir in the pepper powder and cook for few more minutes. Serve hot.