Grate the marrow and squeeze out all the moisture by pressing it between your hands. Put into a large bowl, add the gram flour, ginger, coriander, and chillies, and mix together. Roll the mixture into walnut-sized balls.
Heat the ghee in a kadhai or deep, heavy-based pan over medium heat. Working in batches, gently lower the koftas into the hot ghee and fry for about 2-3 minutes, or until golden. Remove with a slotted spoon and set aside.
To make the sauce, put the ginger, garlic, coriander, and cumin seeds in a small blender or food processor and process until ground. Set aside.
Heat 4 tablespoons ghee in a large, heavy-based pan over medium heat, add the grated onions and, fry for about 3 minutes, or until golden brown. Stir in the ground paste and stir-fry for 5 minutes, adding 1-2 teaspoons water occasionally to prevent the mixture from burning and catching on the base (bottom) of the pan.
Add the tomatoes and cook for a further 5-7 minutes, or until the ghee separates from the paste. Season with salt and add the yoghurt.
Pour in 375ml / 13fl oz (1 1/2 cups) water and simmer for 15 minutes.
Just before serving, add the koftas to the sauce and simmer over low heat for about 5 minutes, or until the koftas are hot.