If u ve oven,just preheat it for sometime. Off it then put the batter inside. Adding baking powder also helps.
Always add salt while grinding n keep inside oven or near gas stove, since these hasten the process of fermenting. Since it is too late to do these, add sour curd n salt n make decisions dosa
Add a big cube of peeled onion in center.put lid & cover it with warm cloth .keep inside kitchen cabinet.will ferment fast.guranteed will fermented fast & no smell of onion too later.just check after 5-6 hrs & remove onion from batter then .keep batter in fridge or use it as required.