Place lamb shanks in a large resealable plastic bag. Whisk together cider vinegar, oil, salt, tamarind concentrate, and garam masala in a bowl, and pour into the bag. Squeeze out excess air and seal the bag. Marinate in the refrigerator for 8 hours or overnight.
Preheat an oven to 450 degrees F (230 degrees C). Grease a foil-lined baking sheet.
Remove marinated lamb shanks and place on the prepared baking sheet. Season all sides with salt. Roast in the preheated oven until well-browned, 15 to 20 minutes. Reserve marinade in the bag.
Combine 1 onion, garlic, ginger, cherry tomatoes, and water in a blender and pulse on and off until smooth. Set aside.
Combine cayenne pepper, paprika, cinnamon, cumin, dried mustard, and black pepper in a small bowl.
Melt clarified butter in a large stockpot over medium-high heat. Cook and stir 1 onion until softened and well-browned, about 30 minutes. Reduce heat to medium and pour in cayenne pepper mixture. Cook and stir until spices are aromatic, about 2 minutes.
Pour marinade from the bag into the stockpot, and stir in tomato-onion mixture and brown sugar. Bring mixture to a simmer. Place lamb shanks in the pan and reduce heat to low. Cover and cook, turning occasionally, until meat is tender and easily pierced with a fork, 3 to 4 hours.
Remove lamb from the pot and cover with foil. Increase heat and simmer the sauce for a few minutes, skimming the fat from the top. Taste and add salt if needed. Serve lamb shanks with sauce spooned over the top. Garnish with cilantro.
