Rava dosas is my favorite! It requires no grinding, the batter once mixed can be used instantly and the chewy-crisp texture of the dosas is superb.
The most important factor in the making of rava dosa is adjusting the heat control and the consistency of batter. An iron tawa requires higher heat as compared to non-stick pan. I have used an iron tawa and so adjust heat according to the pan you use to prepare dosas. You need to be around the stove while preparing these dosas. To get crispy dosas cook the dosas on a slow flame.
Stir the batter well before spreading the dosas as the solids always settle at the bottom. The batter should be thin as rava soaks up lot of water while cooking. So try this and let me know your experiences 
Ingredients of Onion Rava Dosa
2 cups semolina/rava
1/2 cup rice flour
1/2 cup maida (all-purpose flour)
1 1/2 tsps cumin seeds
1 tsp grated ginger
curry leaves – few (optional)
finely chopped green chillies – 1 or 2
2 onions, finely chopped
salt to taste
water – 6 cups (approx)
oil as required
Method of Onion Rava Dosa
1. Combine all the above ingredients except oil and let the rava dosa batter sit for half an hour to an hour.
2. In a 8 – to 10 – inch nonstick griddle or frying pan heat a small amount
of oil over medium heat until a light sprinkling of water sputters and crackles on surface of pan.
3. Before frying each dosa, stir batter well. It should be thinner than crepe batter, so it will spread easily on the griddle. It might have thicken after a while as rava absorbs the water. At the time of making dosas add more water such that it is of pouring consistency (like thick buttermilk).
4. Ladle about 1/3 cup batter onto the hot, griddle in a circular motion, starting from the outside and moving in, to make a thin pancake.
5. Fill the gap in the middle with dosa batter. Don’t drop batter like a thick lump but pour all over, evenly, filling the gaps. Sprinkle chopped onions on the top and let it cook on a low flame. Pour 1-2 tsps of oil like drops along the edge of the dosa and the gaps in the middle.
6. Increase heat to medium flame and let the dosa cook for around 3 mts. At this point increase flame to high and let the dosa roast for a half a minute or so. If you want a slightly softer dosa, remove the dosa at this point.
7. If you want more crispier dosa than let it cook till it reaches a nice golden brown colour. Serve hot with coconut chutney, potato bhaji or sambhar.