Heat the oven to 350 degrees F. In a Dutch oven or high-sided skillet, heat oil over medium-high heat. Season the lamb shanks with salt and pepper. Working in batches, brown the lamb on all sides; transfer to a plate. Pour off all but a few tablespoons of fat from the pan.

Reduce the heat to medium and add the onions. Cook, stirring often, until the onions are tender and just beginning to brown. Stir in the garlic, ginger, fennel, coriander, the whole cumin seed, the mustard seed, the cardamom and the cayenne and cook one minute more.

Return the lamb shanks to the pot, and add the tomatoes and enough chicken broth to barely the shanks. Bring to a gentle simmer on the stovetop then transfer the pot, uncovered, to the oven.

Braise in the oven, basting and turning the meat occasionally. Adjust the heat as necessary to maintain a gentle simmer (barely bubbling). The lamb is done when it's tender enough to cut with a fork and easily comes away from the bone, about 2-1/2 hours.

Transfer the lamb to a plate, let the liquid cool, and spoon off any fat. (For a smooth, refined sauce, strain the liquid and return to the pot.) Bring the sauce to a simmer and reduce until it thickens enough to coat the back of a spoon. Return the lamb to the pot.

When ready to serve, warm the meat in the sauce, basting frequently. Stir in the cilantro and toasted cumin just before serving.

Make Ahead Tips: The lamb is even better made a day or more ahead. Prepare the recipe through the step of reducing the sauce, then return the lamb to its sauce and refrigerate, covered, for up to five days or freeze for up to three months. To serve, reheat the lamb shanks in their sauce, and stir in the cilantro and toasted cumin.