In a tempering pan, add oil, cumin, chopped ginger followed by green chillies. Saute them together for few seconds. Transfer this to mashed potato. Add salt, turmeric powder, chopped coriander and curry leaves too. Mix well and keep it aside.
When the stuffing is ready, prepare the batter. In a wide bowl take gram flour, rice flour, salt, turmeric powder and red chilly powder. Give a quick mix. Adding water incrementally, prepare medium thick batter.
Next peel off the onions and cut each one in to 3 thick roundels. Separate the rings. Take a pair of a big and small onion ring.
Keep the small onion ring inside the big one and fill the prepared stuffing between rings. Get a batch stuffed onion rings like this.
Next heat enough oil in a heavy bottom wok / kadai. Carefully dip the stuffed onion ring in to gram flour batter such that batter coats the ring. Drop it in to hot oil.
In medium flame deep fry these rings until golden brown. Once done, carefully remove from oil and drain over kitchen tissue. Repeat the process with all the onion rings.
Spicy stuffed onion rings are ready to serve with a cup of coffee/tea.