Heat coconut oil in a heavy-bottomed pot over medium heat. Add mustard seeds, fennel seeds, and curry leaves. Cook until they begin crackling.
Add sliced onions, a pinch of salt, and green chillies. Sauté until onions turn golden brown.
Mix in crushed garlic, grated ginger, turmeric powder, black pepper, Kashmiri chilli powder, ground coriander, and garam masala. Cook for 3-4 minutes until fragrant.
Add beef chunks and coat well with the spice mixture. Sauté for 7-8 minutes until beef is browned on all sides.
Stir in tamarind paste, chopped tomatoes, and water. Bring to a boil.
Reduce heat to low, cover, and simmer for 1-1.5 hours until beef is tender and sauce has thickened.
Adjust seasoning with salt and serve hot with rice or bread.