Place ginger, coriander, cumin, tomato sauce & 3 tb water in a blender & blend till smooth. Break potatoes into bite sized pieces. Heat oil (I use ghee) in a large pot. When hot, throw in fennel, fenugreek & mustard seeds. After 20 seconds, add red chilies. As they darken, put in paste from blender. Fry for 5 minutes, stirring frequently. Put in the potato pieces & fry for 3 to 5 minutes. Add 1 1/2 c warm water. Bring to a boil. Add salt & lemon juice. Simmer for 20 minutes. Before serving, sprinkle with garam masala. Madhur Jaffrey, "An Invitation to Indian Cooking.