In a large bowl, mix together rice, lentils, split peas, and mustard seeds. Cover completely with hot water, to about 1″ above the rice mixture level. Allow to soak at room temperature overnight.
In the morning, strain mixture, reserving about 1 cup of the rice water. Place strained rice and beans mixture into a food processor with about 1/4 cup of the reserved rice water, puree for about 5 minutes, or until very, VERY smooth.
Transfer rice batter to a large bowl, stir in yeast and salt. Add enough of the rice water (if needed) to make a very thick batter, about the consistency of THICK pancake batter.
Cover bowl with a clean tea towel, allow to ferment in a warm place for a few hours.
Once you’ve fermented the mixture, give it a good stir – some separation will have occurred. (Note: it’ll smell awful. That’s ok – the finished product does not!)
Heat up a nonstick fry pan (Medium heat), spray with a little pan spray. Pour about 1 cup of rice batter into the middle of the pan, swirling it out to make a “pancake” about 6-8″ across. Allow to cook for a few minutes, until the bottom is set and browning, with air bubbles popping on the top. Carefully flip, continuing to cook on the other side. Cook until batter is completely cooked, all the way through.