Soak the Toor dal & channa dal along with 2 chilies in the water at least for 1 hour.
then drain the water, grind dals with the 2 chilies, salt, and in a mixie
Grind it to a coarse paste and then transfer the mix to a plate, add chopped onions, coriander leaves and mix well.
Make a small ball out of it. Don't Press the ball tightly, it may become hard.
Heat the oil and fry the vadas.
Notes
Grind the batter by adding a little water, if the vada drinks too much water adjust it by adding a tsp of rice flour into it. Fry it in medium flame so that the vada turns crispy and also it will not get burnt.
Recipe by Prema's Culinary at https://www.premascook.com/2020/03/rasam-vadai-recipe-2-types-rasa-vada-makingvideo.html