Preheat the oven to 250 degrees. In a large enameled cast-iron casserole, heat the oil until shimmering. Season the lamb with salt and black pepper and add it to the casserole. Cook over moderately high heat, turning the pieces once or twice, until deeply browned, 15 to 18 minutes. Transfer the lamb to a platter.

Add the onions, celery, and carrots to the casserole. Cover and cook over low heat just until softened, about 5 minutes. Add the tomato paste and cook over moderately high heat, stirring until the paste is lightly browned, about 5 minutes. Add the wine and cook, scraping up any browned bits stuck to the pot, until nearly evaporated, about 5 minutes. Add the water, season with salt, and bring to a boil. Return the lamb and any accumulated juices to the casserole. Cover and braise in the oven for about 2 1/2 hours, until tender.

Transfer the lamb to the platter. Remove any fat and gristle and coarsely shred the meat. Strain the broth and discard the solids. Skim the fat from the surface of the broth. Return the broth to the casserole and boil until reduced to 4 cups, about 30 minutes.

In a small bowl, mix the butter with the flour to form a paste; whisk it into the broth and simmer until thickened, about 5 minutes. Return the lamb to the sauce. Add the olives, red peppers, and tarragon, and keep warm.

Bring a large pot of salted water to a boil. Add the couscous and cook until tender, about 5 minutes. Drain the couscous and transfer to shallow bowls. Spoon the lamb ragout over the couscous and serve.