Preheat oven to 325 F.
Season lamb with 1 tsp each salt and pepper.
Melt butter in a large oven-safe pot over medium heat. Add onions and cook, stirring occasionally, until they start to brown around the edges, 6-10 minutes.
Add garlic, cinnamon sticks, cumin seeds, ginger, and cayenne. Cook, stirring, for 1 more minute.
Add tomatoes and their juice, rhubarb, broth, raisins/currants, and brown sugar. Increase heat to high and bring just to a boil, then turn off the heat.
Add the lamb to the stew, tucking it all under the surface. Cover the pot with its lid. Transfer to the oven. Bake until the lamb is very tender, about 2 hours.
Increase the oven temp to 425 F. Uncover the pot and bake until the sauce has slightly reduced, 30 to 45 minutes. Season with the remaining 1/8 tsp salt and 1/2 tsp pepper. Serve topped with parsley.