In a large bowl combine jowar flour, semolina, salt, curd and water. Mix it well and rest the batter for 20 - 25 minutes ( not more than that)
While the batter is resting do the tempering. Heat a small pan and heat oil. Add mustard seeds and let them pop. Now add chana dal and urad dal. Fry till dal turns light brown. Add cumin, ginger, finely chopped green chili and curry leaves. Fry for a minute and add a pinch of asafoetida and turn off the stove.
After 30 minutes add this tempering to the jowar-rava mixture. Mix it well. Check for the batter consistency and add some water if needed. The batter should be similar to idli batter. Slightly thick, but pourable.
Add eno and one teaspoon of water on top of the eno to activate it. Mix it. Grease your idli plates with oil/ghee/cooking spray.
After the eno is added, you will almost immediately see bubbles in the batter. Immediately pour a ladle full of batter in to idli molds and steam them.