hi dear..best is to heat the tawa on high heat and once it is heated..lower the flame. Next clean the surface of the tawa and slowly spread the batter on the tawa..i mostly use a spoon for that. Just ensure that you start spreading as soon as the batter touches the surface of the tawa.
When we add onion etc in rawa dosa batter,it s not easy to roll,like other dosa. So make batter s bit watery then with a big spoon just spread spoonfulls on the tawa. Cover the big holes with batter. Or make only batter. Cut veg & keep aside. After rolling dosa,put the veg on top. That s also good.
The trick is to heat the pan, lower it before pouring the dosa batter... And make the batter watery.. once this is poured, swirl the dosa pan so that you get a circle, or take the batter in a ladle and pour the batter inside out.. that way it will not mass up when you spread it using a ladle