Clean vazhaipoo and pressure cook for 1 whistle with a cup of water and turmeric powder.
Allow it to cool down then grind it to a semi coarse paste in batches, transfer to a mixing bowl.
Soak channa dal for 30 mins then grind it with ginger, garlic and fennel seeds to a smooth paste.
Sprinkle just a few drops of water if needed.
Now in a bowl add chopped onions, coriander leaves, roasted gram dal powder, rice flour and both the ground pastes - ground vazhaipoo and channa dal paste.
Do not add water. Mix well and make small patties / vadai shapes and keep aside.
You can also refrigerate the patties for 10 mins to avoid the vadais drinking much oil.
Then heat oil in a kadai and deep fry them to golden brown.