Take the wheat flour, rice flour, and salt in a mixing bowl. Finely chop the onions, green chillies, ginger, and curry leaves. Heat 1 tbsp of oil in a small pan; add mustard seeds, urad dal, chana dal, and cumin seeds. When the dal turns golden, add the chopped onions, chillies, ginger, and curry leaves. Saute until the onions turn soft and translucent.
Add the sauteed onion-tempering mixture to the flour in the bowl. Mix well. Add about 1 cup of water and whisk thoroughly using a wire whisk to form a smooth, lump-free paste.
Add the remaining water (about 1 to 1 1/4 cups more) and whisk into a smooth batter that is slightly thinner than regular dosa batter. The batter should flow easily so you can make thin, crispy dosas. (If batter is too thick, dosa becomes dense; if too watery, it may tear while flipping).
Heat a dosa tawa or cast iron skillet on medium-high heat and grease it lightly with a few drops of oil. When the tawa is hot, lower flame to medium. Stir the batter from the bottom, take a ladleful, and pour it onto the hot pan in concentric circles—first filling the outer edges, then filling the center holes. Drizzle 1 tsp of oil around the edges and center.
Cook on medium-high flame until the sides lift automatically and the bottom turns golden brown and crispy. Flip the dosa and cook on medium flame for a few seconds. Remove onto a plate. Before pouring the next dosa, lower the heat, stir the batter thoroughly, and repeat. Serve hot with coconut chutney, tomato chutney, or idli podi!