Drain the water from the soaked urad dal and grind to a coarse paste adding approximately 4-5 tablespoons water.
Place the ground urad dal in a large bowl and add the chopped onions, green chilies, ginger, spinach, coriander, mint, salt, and curry leaves to it. Beat the mixture with you hand for a minute to get a homogenous batter.
Heat oil in a heavy bottomed vessel or kadhai.
Wet your hand in water, take a large ball of the urad dal mixture in your hand and place it on a moistened plastic sheet. Shape it into a flat vadai and make a hole in the center.
Carefully place the shaped vadai in the hot oil. Reduce flame to medium high and fry the vadai in batches. Cook on medium high heat on both sides till crisp and golden brown.