Rinse and soak the dals in adequate water in separate bowls for 6 hours. If you don't have soaking time then soak them in hot water, covered, for 2 hours minimum.
Strain the dals and transfer them to a grinder. Grind them to a slightly coarse paste, gradually adding water (1 tablespoon at a time). The paste should be slightly coarse and thick.
Take out the dal paste in a big mixing bowl. Whisk and fluff it up with your hands or spatula for 3-4 minutes. This step aerates the batter and makes it airy and fluffy, making the dahi bhallas soft and spongy.
Now drop a small ball of mixture in the water-filled bowl; if it floats on the surface, it means it's nicely fluffed up.
Add 1 tsp cumin seeds, grated ginger, salt to taste and ¼ tsp hing to the batter and mix it well. Keep the batter aside.
Heat oil in a kadhai on a medium flame. Wet your hand and Carefully drop the dahi Bhalla batter in hot oil. You can shape them as per your choice, round or flat disc shape.
Fry the bhalla on a medium flame until they turn golden brown from all sides. Fry the bhallas in small batches, do not overcrowd the kadhai.
Meanwhile, add salt and hot water to a big bowl (sufficient to soak the bhallas). Mix it well and keep it aside.
Drain and transfer the fried Bhalla to the bowl filled with hot water. Submerge them in the hot water and let them soak for 30 minutes, minimum.