1. Tamarind Paste:
The amount will vary depending on the brand you use and the taste. I use a Tamicon brand and for us 1 tsp of this works fine. You can go the traditional route and use regular tamarind (a lime sized ball should suffice), drop it in hot water (1 cup worth) and extract all the juice. Discard the tamarind and use this water instead of regular water in this recipe.
2. Sambar Powder:
Like any dutiful daughter, I tend to follow my mom's recipe for Sambar powder
. So for us with this powder, 1 tbsp works great. But if you have your own recipe for the powder or are using store bought powder, you will have to adjust the amount. Start with 1/2 tbsp and work around your requirements.
The addition of coconut takes this humble stew to greater heights. Frozen coconut also works. But in case you do not want to use coconut (health or unavailability reasons), use 1 tbsp Rice flour instead. This is needed to thicken the stew. My mother never used coconut but always rice flour in this recipe and it still would taste good.
Almost any kind of vegetables can be added to enhance the stew. More often than not, you would need to parboil the vegetables separately with little salt before pouring the tamarind.You can optionally saute it in some little oil to enhance the taste before boiling. Vegetables like Okra, Brinjal, Drumstick, Cauliflower, Potato, Carrot, Bell Pepper etc can be used.