Season the lamb shanks with salt, pepper, garlic powder, cumin, and rosemary. Place them 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 shanks finish cooking, set the slow cooker to WARM.
Transfer the cooked shanks to a plate. Strain the cooking liquids into a glass measuring cup. Return the lamb shanks to the slow cooker pan (carefully wipe it with warm, moist paper towels if needed) and cover to keep the shanks 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 the liquids are reduced into a rich gravy, 10-15 minutes.
Transfer the lamb shanks to plates, spoon the gravy on top, and serve.
Video
Notes
There's no need to brown the shanks on the stove before adding them to the slow cooker. I've experimented with browning them first and have found that this extra step is not worth the trouble. The shanks are excellent, with or without browning.
The shanks I typically use are imported from Australia or New Zealand. Local ones are often bigger. If yours are much bigger, you'll need to increase the cooking time, adding 30 minutes on HIGH or one hour on LOW.
It's important to skim the fat from the top of the cooking liquids with a spoon before attempting to reduce them. Otherwise, the oil can separate. It's easier to separate the fat from the cooking liquids if you strain them into a measuring cup and place the cup in the freezer for 10-15 minutes. The fat will float to the top and slightly harden, making it easy to remove it.
After removing the fat, cook the liquids patiently over medium heat just until thickened, then immediately turn the heat off. This can take as long as 15 minutes. You can speed up the process by mixing a teaspoon of cornstarch with a tablespoon of cold water and stirring this cornstarch slurry into the cooking liquids.
You can keep the leftovers in the fridge, in an airtight container, for up to 4 days. Reheat them gently, covered, in the microwave at 50% power, turning them midway through reheating.