Preheat oven to 350 degrees. In a large bowl, toss lamb with flour; season with salt and pepper. In a Dutch oven or other large, heavy-bottomed saucepan (at least 5 quarts), heat oil over medium-high. In two batches, cook lamb until browned on all sides, about 5 minutes per batch; transfer to a plate.
Reduce heat to medium, add onion, garlic, and tomato paste to pot; season with salt and pepper. Cook, stirring frequently, until onion has softened, 3 to 5 minutes. Add cinnamon sticks, 4 cups water, and reserved lamb (with any juices that have accumulated); bring to a boil.
Cover pot and transfer to oven; cook until lamb is just tender, about 1 1/2 hours. Add squash, return to oven, and cook until squash is tender, about 30 minutes more. Remove cinnamon sticks, stir in dried plums, and season with salt and pepper. Serve with cilantro and couscous.