You'll need moong dal for this filling. This is the split yellow mung bean without the skin on. Soak the mung for at least two to three hours before you cook it, and, if you have the time, overnight. If soaking for a shorter time, use warm water. You want it to soften as much as possible in the water. Then blitz the dal slightly so it breaks into smaller bits but doesn't become a paste--some of the lentils should remain whole. This helps them cook fast on the stove.