Soak chana dal for 3 hours. First grind ginger, garlic, chillies and fennel coarsely.
Then drain chana dal completely and add it in two batches and grind coarsely. Not too smooth not very coarse too. Refer picture. Few chana dal can be whole, no issues.
Mix finely chopped onion, coriander, mint, salt, rice flour and curry leaves.
Heat enough oil in kadai.
Keep a bowl with water and wet both hands. Make equal sized balls first for uniform sized vadas. Shape each ball to flat patties of medium thickness. Edge thin, thicker in the middle just like how a turtle shell would be looking.
Deep fry in hot oil until golden in colour, flipping once in between.
When you drop oil, heat should be high. Simmer after you drop all vadas and cook in medium flame until done.
Drain in paper towel. Repeat to finish.