Rinse the pork loin and dab dry with a paper towel.
Combine the garlic powder, ginger, thyme, Kosher salt and pepper in a small bowl. Rub the spice mixture into pork loin.
Heat vegetable oil in a large skillet over medium-high heat. Sear pork on each side until golden brown, 1-2 minutes per side.
Transfer the pork loin into the slow cooker and top with the sliced onions.
In a medium bowl combine the apple juice, Worcestershire Sauce and brown sugar. Pour over the pork roast.
Cook on low for 4-5 hours. Test with an instant read thermometer for 135°F.
Remove the pork and tent with foil to keep warm while resting. Meanwhile, prepare the gravy.
Onion Gravy:
Pour juices through a fine-mesh sieve to remove any large pieces, reserving onions for serving. Transfer liquid into small saucepan and bring to a low simmer.
Combine the cornstarch and water until smooth (slurry), slowly add to saucepan, whisking constantly to make gravy. Remove from heat.
Slice the pork and top with reserved onions and gravy.
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