Rinse the rice, horse gram, and black urad separately until the water runs clear. Now add the ingredients to a bowl with sufficient water and soak for atleast 8 to 10 hours.
After 8 hours, drain the water. Add ¼ cup of water to the blender and black urad and half of the horse gram mixture and grind it. Add water as required.
Transfer the batter to a bowl and grind the remaining horse gram and rice together with green chilies. Add water as required while grinding. Overall I used 1.5 cups of water while grinding.
Transfer the batter to a bowl, rinse the mixer jar with ¼ tbsp, and add it. Add salt and mix well.
Make the adai
Add finely chopped shallots and spinach to the batter and mix well. If needed, add ¼ cup of water.
Heat a well seasoned cast-iron pan, and when it is hot, reduce the heat to medium-low. Take a ladle full of batter, pour it on the pan, and spread it gently.
Spread some oil around the edges let the adai cook. The cooking timing differs depending upon the thickness of the batter and veggies added; when the adai is fully cooked, the color will change. You can flip the adai then.
When the top part is fully cooked, loosen the edges, flip the adai, and cook for 30 to 45 seconds. Now flip the adai and remove it from the pan. Similarly, make the rest of the adais with the remaining batter and serve hot.
If the batter is thick, thin it down by adding 2 to 4 tbsp of water. Thicker batter will yield thick adai and if you prefer that way, go for it.
I have mentioned the water measure I used; please adjust as required. The amount might vary as per the rice quality.
Adjust the salt and green chilies to taste. Instead of green chilies, you can use dried red chilies too. In that case, soak it along with rice.
If you are not following a vegan diet, you can use ghee to prepare the adai.