Place lamb in a 4-quart or larger slow cooker. Mix oil, cumin, coriander, cayenne, salt and pepper in a small bowl. Coat the lamb with the spice paste and toss to coat well. Top with onion.

Bring tomatoes, broth and garlic to a simmer in a medium saucepan over medium-high heat. Pour over the lamb and onion. Cover and cook until the lamb is very tender, 3 to 3 1/2 hours on high or 5 1/2 to 6 hours on low.

Skim or blot any visible fat from the surface of the stew. Mash 1/2 cup chickpeas with a fork in a small bowl. Stir the mashed and whole chickpeas into the stew, along with spinach. Cover and cook on high until the spinach is wilted, about 5 minutes.