Slightly roast the oats and semolina in a pan. Grind them by adding enough water to make a batter of medium thick consistency.
To make the dosa:
Add the bicarbonate of soda, salt and mix well to get a pouring batter consistency. Heat a tawa, slightly grease with oil. Pour ladle full of batter and swirl it around. Sprinkle the chopped onion and coriander leaves on top.
Once cooked, turn to the other side and cook for another 2 minutes. Repeat with the batter to make the dosa.