In a large bowl, mix together the yoghurt, tomato paste, paprika, cumin, coriander, cinnamon, salt, pepper, garlic, and onion. Add the lamb to the bowl, making sure it is coated evenly with the marinade. Cover the bowl with plastic wrap and refrigerate for at least 2 hours, or overnight for best results.
Preheat your oven to 325°F. Remove the lamb from the marinade and place it in a large Dutch oven or roasting pan. Drizzle with olive oil and cover tightly with a lid or foil. Bake for 3 to 4 hours, or until the lamb is tender and falls off the bone.
In a medium saucepan, bring 2 cups of water to a boil. Add the bulgur and a pinch of salt, reduce the heat to low, and cover the pan. Cook for 15 to 20 minutes, or until the bulgur is tender and the water is absorbed.
In a small bowl, mix together the softened butter and chili flakes until well combined.
To serve, spoon the bulgur onto a large platter. Place the lamb on top of the bulgur and drizzle with the chili butter. Serve with a dollop of yoghurt on the side.