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1. In a large bowl, combine the beef cubes with turmeric powder, cumin powder, coriander powder, and salt. Mix well to ensure the beef is evenly coated with the spices.
2. Add the white vinegar and tamarind paste (if using) to the beef. Cover the bowl with plastic wrap and let it marinate in the refrigerator for at least 2 hours, or preferably overnight.
3. Heat the vegetable oil in a large, heavy-bottomed pot or Dutch oven over medium heat.
4. Once the oil is hot, add the mustard seeds and fenugreek seeds. Sauté until they start to crackle and become fragrant, about 30 seconds.
5. Add the finely chopped onions and sauté for about 10-15 minutes until golden brown.
6. Stir in the minced garlic, grated ginger, and slit green chilies. Cook for another 2-3 minutes until the raw smell of garlic disappears.
7. Add the marinated beef to the pot and stir well. Cook for about 5-7 minutes, allowing the beef to brown on all sides.
8. Pour in the tomato puree and mix well with the beef.
9. Add the brown sugar or jaggery and sprinkle in the garam masala. Stir and let it cook for another 5 minutes.
10. Pour in enough water to cover the beef (about 2-3 cups). Bring to a boil, then reduce the heat to low.
11. Cover the pot and let it simmer for about 1.5 to 2 hours, or until the beef is tender and the sauce has thickened. Stir occasionally.
12. For a thicker sauce, remove the lid during the last 30 minutes of cooking.
13. Once the beef is tender, taste the curry and adjust the seasoning as needed. Add more salt, sugar, or vinegar according to your preference. For extra heat, add more green chilies or a pinch of red chili powder.
14. Remove the pot from heat and let it sit for about 10 minutes before serving.
15. Garnish with freshly chopped cilantro and serve hot with steamed basmati rice, naan, or your favorite Indian bread.

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