Steam cook the mixed vegetables and the potatoes till well done. Mash them very well and keep aside.
In a largish wok, heat the oil. Add the ginger and garlic pastes and sauté taking care to see it doesn’t burn. Add half the onions and sauté again it is soft and translucent. Add the chopped tomatoes and cook them till they’re soft and mushy.
Use your potato masher, or a wooden spoon, to mash the onion-tomato mixture further. Cook until the oil appears on the edge.
Add the turmeric, chilli, coriander, cumin and garam masala powders. Cook over medium heat for a couple of minutes, stirring often, until the raw smell of the spices disappears. Add the mashed vegetables, salt and about half a cup of water. Cook for another 5 to 10 minutes until everything blends into a homogenous thick gravy-like consistency, adding a little more water, if necessary.
Add the chopped coriander and a couple of tbsps of butter, mix and serve hot.