Soak the poppy seeds, the yellow and black mustard seeds in warm water for about 20minutes. Drain the water and make a fine paste with half of the green chilies and very little water. Keep aside.
Heat 2 tsp oil in a pan, add the chopped onions, garlic and green chilies and saute for 4-5 minutes. Add the mustard-poppy seed paste and a sprinkle of salt. Cook on low flame for another 5 odd minutes till the moisture has almost evaporated. The stuffing is ready.
In the meantime, wash the flat beans and pat dry. Top, tail and string each of the beans. Gently slit along the edge of the bean with a sharp knife, creating a kind of pocket.
Take a spoon of the stuffing and carefully fill inside the bean as shown in the picture. Repeat the same for all the beans and keep aside.
Take all the ingredients for the batter in a bowl, add a little water to make a thick batter.
Heat oil in a frying pan, dip each of the stuffed beans in the batter and fry till golden. Serve hot!