Nemu is lemon in Assamese. And Tenga means sour. It’s more like Lemon Shorba . 
Some of the Assamese households make it and either use lentil fritters (Lentil pakodas) or use fish. Sour curry is a regular item in an Assamese Thali. The sour curry is made at times using lemon, often it’s made with tomatoes and at times with Thekera Tenga (this is of the same family as Kokum). At times we also make the sour curry with Star Fruit, or Ou Tenga (Elephant Apple) and sometimes with oranges as well. I use Meyer’s Lemon (a type of lemon found here in US) at times as this lemon has an orangey lemony flavor. In Assam we get a variety of lemon called Tulia Tenga which has close resemblance in shape and taste to our Meyer’s lemon.
In Assam a variety of lemon called Kaji Nemu is used. It’s a green colored oblong shaped lemon. More so resembles the limes we get here.
Lemon Curry/Shorba Recipe:
For this curry I used our regular round yellow lemons. For the curry or shorba ( for 2 regular size bowls) you would need
Ingredients:
Concentrated lemon juice squeezed from ¾ of a regular sized lemon
1+1/2 cup of water
1 teaspoon of black mustard seeds
1 green chili slit
2 dry red chilies
1 teaspoon of cooking oil
A pinch of salt and pinch of turmeric
1 teaspoon of sugar
Method:
Heat oil in a pan. Add the mustard and dry red chilies and allow them to crackle.
Now add the lemon juice, salt, turmeric and the 2 cups of water and the sugar.
Bring it to a good boil. Allow some of the water to evaporate. Then add the green chili.
I like my gravy albeit watery. But if you want thicker gravy you can also add a teaspoon of cornstarch to thicken it. Add the cornstarch to 2 tablespoon of water and mix well and then fold it in the lemon gravy.
Lentil Fritters Recipe:
Can be made with any lentils. I used Chana Dal or Bengal Gram Dal. For 6 badis/fritters/pakodas you would need:
Ingredients:
½ a cup of Bengal Gram Dal. Soak them in water for about 3 hours and then make a coarse paste
3 sprigs of cilantro finely chopped
A pinch of salt and turmeric
¼ of a medium sized onion finely chopped
½ of a green chili chopped
Method:
Mix all the above ingredients and then fry the pakodas in hot oil.
You may add the pakodas to the lemon curry just before turning the heat off so that the pakodas moisten by absorbing the gravy.
– Courtesy Smritilekha Das