Measure and add the lentils and rice in two separate bowls.
Rinse the lentils under running tap water, until the water appears clear. Soak the lentils with whole dry red chillies in sufficient water for 3 hours.
Rinse the rice and soak it for 3 hours.
Drain the water used for soaking, add the lentils to a mixer grinder jar.
Grind the lentils and red chilies to a coarse consistency (Refer pic below). Add water at regular intervals and check consistency.
Remove the ground lentil mixture into a bowl.
Next, in the same mixer grinder jar, grind the soaked rice with some water. Add the rice batter to the ground lentils. Mix well using hands.
To the ground adai batter, add salt to taste and hing (asafoetida). Rest the batter for 30 minutes. If you are not preparing adai immediately, store the batter in refrigerator.
Remove a portion of the batter in a separate bowl. To this, add finely chopped onions and roughly torn curry leaves. Mix well, check consistency. If it appears too thick, add some water and mix.
Heat a cast iron griddle or non-stick dosa pan. Grease it with few drops of oil. Pour a ladle of adai batter and spread it clockwise until it forms a circle.
Pour oil on the sides and center and let it cook on medium to low heat. When the edges appear brown and crispy, flip / turn, cook on the other side until crispy and golden brown.