Wash and soak ragi rava for about 30 mins. Drain completely and squeeze off the excess water using your hands. Set this aside.
Heat a pan with oil. Add cumin, mustard, chana dal, urad dal and cashew nuts. Fry them until the lentils turn golden.
Add hing, onions, curry leaves, ginger and green chilies. Fry them until the onions turn transparent.
Transfer the rava and saute for a while for about 3 to 4 minutes until you begin to smell the rava good.
Meanwhile stir water and salt in a pot. Taste test the water. It should be slightly salty. Bring this to a boil. Turn down the heat and slowly pour the water with one hand, while you stir the rava with another hand. Be careful as the water can splash at this stage.
Cover and cook on a low to medium heat until all the water is absorbed and cooked through. If you think it is still uncooked, simply add more boiling water. Stir in between a few times. Turn off and let rest for 10 to 15 mins.
Sprinkle some lemon juice and garnish with coriander leaves. Serve ragi rava upma hot.