Season the lamb shanks with salt, pepper, garlic powder, cumin, and rosemary. Place the shanks in your slow cooker.
Cover and cook on HIGH for 4 hours or on LOW for 7 hours, until the lamb is very tender.
When the lamb shanks finish cooking, set the slow cooker to WARM.
Transfer the cooked lamb shanks to a plate. Strain the cooking liquids into a glass measuring cup, then return the lamb shanks to the slow cooker pan (carefully wipe it with warm moist paper towels if needed) and cover, to keep the meat warm while you thicken the gravy.
Use a spoon to skim the fat layer off the top of the cooking juices. Pour them into a medium saucepan. Heat over medium-high heat until almost boiling but not quite, then turn the heat down to medium and simmer, whisking occasionally, until liquids are reduced into a rich gravy, 10-15 minutes.
Transfer the lamb shanks to plates, spoon the gravy on top, and serve.
There's no need to brown the lamb shanks on the stove before adding them to the slow cooker. I've experimented with browning them first, but I have found that the extra step is not worth the trouble. The dish is excellent with or without browning.The lamb shanks I typically use are imported from Australia or New Zealand. Local lamb shanks are often bigger, and if yours are much bigger, you might need to increase the cooking time - try adding 30 minutes on HIGH or one hour on LOW.