In a medium size bowl, soak moong dal in water for at least 4 hours or overnight. Make sure that the water covers the beans by at least 1~1½ inch.
Once soaked, drain the water completely and transfer the beans to a food processor or blender. Add ginger, green chilies, salt and grind to a coarse batter. Transfer the batter to a medium size bowl and add cumin seeds, chopped cilantro, curry leaves and onions, if using. Mix well. Batter is ready to make vada.
Heat oil in a medium size pan on medium-high heat for deep frying. Test if the oil is hot enough by dropping a bit of batter. If it sizzles and comes up to the surface, it is ready to go. Take about 1 tablespoon of batter and flatten it. Place it either on slightly damp plastic Ziploc bag or your palm. If you want to make garelu, then make a small hole in the middle with your finger. To make vada, leave it as a patty.
Gently slide the batter into the hot oil. Fry the vada until golden brown on both sides, about 2~3 minutes per side. Depending on the size of your pan, you can fry multiple vada at a time. Remove fried vada using a slotted spoon onto a paper-towel lined plate. Repeat with the remaining batter. Serve hot with any chutney.