In a small skillet, toast the cumin, fennel, and coriander seeds over moderately low heat until just fragrant, about 1 minute; let cool completely. Grind the seeds in a spice grinder, then transfer to an airtight container. Stir in the paprika, black pepper, sugar, allspice, and cayenne and cover the merguez spice blend until ready to use.

Preheat the oven to 325 degrees. Season the lamb with salt and dredge in the flour, shaking off the excess. In a large enameled cast-iron casserole, melt the butter in the olive oil. Add 4 of the lamb shanks and cook over moderately high heat, turning frequently, until well browned all over, about 8 minutes. Transfer the lamb shanks to a plate and repeat with the remaining 4 shanks. Discard all but 1 tablespoon of the fat from the casserole.

Reduce the heat to moderate. Add the onion to the casserole and cook, stirring, until lightly browned, 5 minutes. Stir in 4 tablespoons of the spice blend along with the tomato paste and cook until aromatic, 2 minutes. Add 1 cup of the chicken stock and scrape up the browned bits from the bottom of the pot. Stir in the tomatoes.

Return the lamb to the casserole and pour in any accumulated juices. Add the chickpeas. Pour in just enough of the chicken stock to cover the meat and bring to a simmer. Cover the casserole and transfer to the oven. Braise the shanks for 1 1/2 hours, or until the meat is tender and pulling away from the bones.

Transfer the braised lamb shanks to a large, deep platter. Using a large spoon, skim the fat from the surface of the sauce. Spoon the sauce over the shanks and serve, passing harissa at the table.

The braised shanks can be refrigerated in the sauce for up to 3 days. Reheat gently before serving.

Harissa is a North African chile paste. It is available at specialty shops.