Soak the mung beans overnight in about 4-5 cups of water.
Boil the mung beans in 4-5 cups of water and keep aside. (Do not drain the water). Takes about 30 mins in a pot. Or 15 mins in a pressure cooker.
Heat oil in a pan on medium heat and add the mustard seeds and allow to splatter.
Once done add thinly sliced shallots, ginger, garlic and chillies. Saute till the onions are translucent.
Add the prawn and fish pieces to the pan and add soy sauce. Cover them with the onion soy sauce mixture and allow to cook for about 2-3 minutes.
Add the boiled mung beans and cut spinach leaves to the pan or add the ingredients of the pan to the pot containing boiled mung beans and bring to a boil.
Squeeze the juice of half a lemon in the pot to enhance the flavors and add salt as per taste.
if you find the soup thick add water as per your requirement.
One can also add noodles in the soup and make a complete meal out of it.
One can also serve the soup (thicker version) with cooked rice or one can enjoy the soup plain.