If u don’t get coarse ground rice flour take normal rice flour and sieve it through the finest sieve and take whatever is retained on the sieve. Heat oil in a deep pan and add mustard seeds and when they crackle add urid dal and fry till browned. Now add water and salt and bring to boil. Add rice flour slowly, mixing well. Bring to boil and then reduce heat and cook on medium heat till all water is absorbed. Remove from flame. When the rice is cool make walnut sized balls out of it and steam them or cook them on an idli stand. Serve hot with coconut chutney.