For Making Dahi Vada: Make a thick paste by grinding soaked urad dal (try not to add too much water else it will be difficult to shape the vada later). Add Salt and Rava. Mix and keep aside.
In a big wide container mix curd, water, grated ginger, grated mango ginger(if using). Keep aside.
In a pan heat 1 tbsp oil. Add mustard seeds, Cumin, Urad dal, Curry leaves, dry red chilies, split green chili. When the mustard seeds start to crackle and the tampering is aromatic switch off the gas. Let it cool down for 5 minutes and then add to the curd mixture.
Next beat the urad dal paste vigorously using your hand. It's the difficult but most crucial step of the process. The more you beat, the more you will incorporate air into the batter. To know the batter is ready, drop a small piece of batter into the water. If it floats then consider it as ready, else some more muscle work.
Heat oil in a kadhai for deep frying the vada. When the oil is medium-high hot take small portions of the batter and shape it. Slowly slide into the hot oil. The oil should not be too hot, else the vada's will remain undercooked.
You would see the vadas will start floating. Drop-in some more shaped vadas into the oil. Don't overcrowd the pan. Fry till they look light golden brown in color on both sides.
Once the vadas are done, remove them from the oil using a slotted spoon and put it immediately into the water. Let them sit in water for 5-8 minutes. Then remove from water and squeeze the vadas in between two palms to drain excess water. Add to the curd mixture.
Repeat the above frying, soaking in water, and finally putting into the curd mixture process for the remaining batter.
Carefully turn the vadas so that they are evenly soaked in curd. While the Vadas soak in the curd mixture, we shall prepare Aloo Dum.
_For Aloo _Dum: In a pan heat 2 tbsp oil. Add dry red chilies, Tej Patta. Add asafoetida. Add chopped Onion and fry till brown in color.
Add Ginger-garlic paste. Fry till the raw smell is gone. Add Chopped tomatoes. Cook until the tomatoes are soft and mushy. Add turmeric powder, cumin, coriander powder, red chilli powder. Fry until the oil is separated.
Add cubed potatoes. Fry for 6-7 minutes, till the potatoes, are nicely coated with the spices and browned. Add garam masala. Mix well. Add water. Bring the curry to a boil, roughly for 10 minutes.
Serve Dahi bara, topped with warm Aloo dum and garnished with chopped onion, Sev, coriander leaves. And off-course dusted with lots of roasted cumin, red chili powder, and chat masala.