Soak tamarind in 1 cup warm water for 15 minutes and extract thick juice by squeezing and straining.
Heat oil in a heavy-bottomed pan and fry turkey berry vathal until crisp and golden. Remove and set aside.
In the same oil, add mustard seeds, urad dal, cumin seeds, curry leaves, and asafoetida. Fry until fragrant and dal turns golden.
Add sliced onions and minced garlic, sauté until onions are lightly golden.
Pour in tamarind extract and add 1-2 cups water. Bring the mixture to a rolling boil.
Add red chili powder, coriander powder, turmeric powder, and sambar powder if using. Mix well to prevent lumps.
Boil for 5-10 minutes, then add salt and jaggery. Taste and adjust seasoning.
Add the fried vathal back to the curry and simmer on low heat for 10-15 minutes until oil begins to separate on the surface.
If desired, mix rice flour with a little water and add to thicken the curry.
Garnish with fresh curry leaves and serve hot with steamed rice.