This is one of the traditional ways to make this vantha kuzumbu. This is called Field Beans in English. I think you can use surtival instead of mochai.
Soak the mochai overnight.
Cook the mochai in pressure cooker for 3 whistles. After it is cooked drain the mochai and keep aside.
In a pan/kadai pour oil and season with mustard,hing and curry leaves. After it splutters add garlic and onions and saute nicely. Then add the cooked mochai and fry for some time.
Then add the tamarind extract and 1 cup of water, sambar powder/ red chilli powder, salt and mix nicely and allow it boil.
After a while bring it simmer and allow it thicken. Do a taste test and adjust salt and spicy acc to taste. allow it cook till the raw smell goes.
Bfore removing it from fire add a few drops of gigely oil/ sesame oil. Mix nicely and trasnfer to a serving bowl.
Serve with warm white rice. sutta appalam/ toasted papad will go very well with this...:)
this is from my kitchen notes.