Soak the mustard seeds and rice grains in 1/4 cup hot water for 15-20 minutes.
Cook a small sweet potato in the pressure cooker / microwave or on stove top. Peel and keep aside.
Top and tail the tender beans. Chop roughly. In a bowl, mix the mashed sweet potato, chopped beans and peas with 1/2 tsp salt. Keep aside.
In a small mixer jar, add the soaked mustard seeds, rice, coconut, green chilies and grind to a fine paste, adding a few tablespoons of water if need be. Don’t make the paste watery.
Remove this paste into a small bowl. Add 2 tbsp mustard oil, 3/4 tsp salt, 1/2 tsp turmeric powder. Mix well and keep aside.
Prepare the banana leaves by cutting off the thick central rib and cut the leaf into four 6-8” squares. Heat a tava / skillet and warm the leaves on it to make it soft and pliable, so that it does not tear while wrapping the parcels.
Lay out the 4 squares of banana leaf on the counter top. Divide the beans-peas-sweet potato mix between the 4 leaves. Divide the prepared paste between these four portions, placing it on top of the vegetable mix. Fold over the edges one by one to make a snug parcel, securing it with a twine or toothpick, so that the steam remains trapped inside while cooking the parcels.
Place a pan to heat. Grease with 1 tbsp of mustard oil. Place the parcels, cover the pan with a lid and cook on low-medium heat for around 15 minutes. Turn them over and repeat the same on the other side for 3-4 minutes.
To serve, place the parcels unopened along with steamed rice. Untie the parcels just before eating, so that the aromas remain sealed inside.