Cut deep slashes about 2 inches apart across the lamb. Mix yogurt, ginger-garlic paste, lemon juice, ground red chile, chaat masala, garam masala, salt, red pepper flakes, turmeric, black pepper, cumin, and coriander together in a bowl. Whisk until well combined. Apply marinade generously all over the lamb in a dish; turn over and apply more marinade. Cover and refrigerate for 3 hours to overnight. Heat oil in a large cast-iron pot over medium heat. Place the lamb leg gently into the bottom of the pot. Cover and place a heavy weight on top of the lid. Reduce heat to low and cook for 1 hour. Turn leg over and cook for another hour. Continue cooking lamb until liquid is mostly evaporated and meat is tender and golden, about 15 minutes more. Oils should start collecting at the edges of the lamb. An instant-read thermometer inserted into the center should read at least 130 degrees F (54 degrees C). Transfer meat to a serving dish.