Combine the garlic, ginger, chili powder and 1/2 teaspoon salt in a medium bowl. Rub the mixture into the beef and let sit for at least 20 minutes.
Combine the tamarind concentrate with 3 tablespoons boiling water in a small bowl; stir until tamarind concentrate is dissolved, and set aside.
Put 2 tablespoons coconut oil in a large skillet on medium heat. When the oil is hot, add the mustard seeds and curry leaves. Let simmer for 1 minute, then add the onions and chilies, and cook until onions start to soften, about 5 minutes.
Add the garam masala, ground coriander, ground turmeric, powdered black pepper and 1/2 teaspoon salt to the onions. (The mixture will be quite dry.) Cook for another 5 minutes, stirring occasionally, then reduce the heat to low and stir in the tamarind concentrate and yogurt. Cook 3 to 5 minutes, or until the liquid is almost completely evaporated.
Heat the additional tablespoon coconut oil in a second skillet over high heat. Add the beef strips and sear for 30 seconds, turning once.
Using tongs, carefully transfer the beef strips to the skillet with onions, folding them into the onion mixture, and let simmer 3 to 5 minutes, stirring occasionally, until tender and cooked through. Discard the curry leaves; taste and adjust the seasoning, adding more salt if necessary. Garnish with fresh cilantro.