Using a thin knife, cut the lamb off the bones, then cut the meat into 1-inch pieces; discard the bones. Season the lamb with salt and black pepper; refrigerate, if not cooking right away.
Heat a large cast-iron skillet over high heat. Add the oil to the pan and swirl to coat. Add the lamb and cook, stirring occasionally, until well-browned but not cooked through, about 3 minutes. Add the cumin seeds and crushed red pepper and cook, stirring, until fragrant, about 30 seconds. Add the onion and garlic and cook, stirring, until just starting to soften, 1 to 2 minutes. Add the chickpeas and a generous pinch of salt and black pepper and cook, stirring, until the chickpeas are warm, 1 to 2 minutes.
Meanwhile, whisk the stock with the lemon juice and cornstarch in a small bowl. Pour into the skillet and cook, stirring, until the lamb is just cooked through and the mixture is coated in a light sauce, 1 to 2 minutes. Fold in the mint and season with salt and black pepper. Serve right away, passing yogurt at the table.