Its basically a green leafy vegetable which is tangy in taste. In Mumbai its called Ambadi and in Andhra its called Gongura. If both the names don't ring a bell to your vendor, you can ask him for khatta saag, The recipe is just like how you would make spinach, only difference is, you don't add tomatoes here because its already tangy.
Here is the recipe that I follow
Ambadi or gongura leaves (Atleast 3 bunches of the size of spinach that you get in market)
5 garlic cloves
2 green chillies finely chopped
2 onions finely chopped
2 tsp chana dal
5 methi seeds
1 heaped tsp besan (optional)
Pinch of asoefotida
1 tsp mustard seeds
2 tsp vegetable oil
Salt to taste
Wash the leaves thoroughly and bring them to boil in pressure cooker by adding half a cup of water.
Grind the pulp finely using an electric blender.
For the tempering
Heat oil in a kadhai.
Add asoefotida, methi seeds, chana dal and mustard seeds. When seeds start to splutter, add coarsely crushed garlic and let it turn brown.
Now, the optional part, add besan and stir fry till light brown. This helps reduce the tanginess of saag.
Add onions and green chillies and fry till onions turn transparent.
Add pulp of saag to it.
Bring to boil, and add salt to taste.
Enjoy this saag with steam rice. Prawns or fish makes for great accompaniment with it. The recipe of crispy fish fry in this picture can be found here