Blend the tomatoes, onion, ginger and yogurt till smooth. Mix in the coriander and red chilli powder into the paste.
Heat oil in a pressure cooker and splutter cumin seeds.
Quickly add the paste and mix well. Start cooking, stirring occassionally till oil floats on top.
Add 1/2 cup along with salt and bring to boil.
Add in the potatoes and another cup of water. Close the pressure cooker and cook on high for 3 whistles. Simmer and cook for another 5 minutes and switch off. Allow the pressure to go by itself.
Open and sprinkle in the garam masala. Serve hot with sprinkles of coriander leaves.
Notes
Normally this curry is watery, but I added limited fluid to make it thick. Add upto 3 cups of water for more fluid.