In a small bowl, mix the salt, pepper, garlic powder, smoked paprika, and ground cumin.
Pat the beef shanks dry with paper towels. Season both sides with the spice mixture, gently pressing with your hands to ensure the spices adhere to the meat.
Pour the beef stock into the bottom of your slow cooker pan.
Add the beef shanks, layering them.
Cover and cook on LOW for 6 hours.
Heat your broiler on high (450℉). Line a rimmed baking sheet with foil. Remove the shanks from the slow cooker and place them on the baking sheet. They are so tender at this point that some of them might fall apart. That's OK and expected.
Broil the shanks to brown the fat for 1-2 minutes, keeping a close eye on them to ensure they don't burn. Turn the broiler off and set the oven to warm (170℉). Keep the shanks in the oven to keep them warm while you reduce the cooking liquids.
Strain the cooking liquids into a measuring cup. Place the cooking liquids in the freezer for 10 minutes. This will help solidify the fat layer and make it easy to remove it from the top. Remove the fat layer from the cooking liquids using a spoon. Pour the liquids into a medium saucepan.
Prepare a cornstarch slurry by mixing the cornstarch with 1 ½ tablespoons of cold water. Stir the cornstarch slurry into the cooking liquids.
Heat the mixture over medium-high heat, whisking often, until it thickens into a rich glaze. This should take about 10 minutes.
Arrange the shanks on a serving platter or individual plates. Spoon the glaze on top. If desired, sprinkle them with chopped parsley for garnish. Serve immediately.
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Notes
The shanks will start to smell amazing after four hours, but cook them for six hours to ensure they are tender.
Cleaning tip: Reducing the cooking liquids will result in the saucepan needing major scrubbing. Simply fill it with hot, soapy water, let it sit overnight, and then, the next day, scrub it with a stainless steel scrubber (assuming it's not a nonstick pan).
The reason for making a cornstarch slurry instead of stirring the cornstarch directly into the cooking liquids is that cornstarch can turn into clumps.
I experimented with browning the shanks on the stovetop before placing them in the slow cooker. My conclusion is that this extra step is unnecessary. You can achieve the same result with much less effort by briefly broiling the shanks after they are cooked.
If you buy the shanks online, you'll probably get different sizes. That's fine.