Heat oil in a non-stick pan, add cumin seeds and onion seeds. Sauté for half a minute, add onion, garlic, ginger and green chillies and sauté for three to four minutes.
Add capsicum and carrot and continue cooking. Add crushed chana dal, dried mango powder and salt and mix. Set aside to cool.
Bring one cup water to boil in a thick-bottomed non-stick pan. Add one-teaspoon salt and oil. When the water starts boiling, add rice flour in a continuous flow, stirring rapidly to avoid lumps. Cook for three to four minutes, stirring all along.
Remove on to a plate, cover with a damp cloth and let it sweat for a few minutes. Knead with your palm to a smooth dough, cover and keep. Divide the rice dough into sixteen to eighteen equal portions and make them into balls.
Lightly oil your palm and spread each ball to approximately three inches round disc, by pressing with your fingers to as thin as possible. Place sufficient filling and fold into half moon shape. Press the edges firmly with your fingers to seal.
Repeat with the rest of the dough. Heat water in steam pot and steam vegetable crescents in small batches for about ten to twelve minutes or till completely cooked. Serve straight from the steamer with a hot and spicy sauce of your choice.