Season the brisket on both sides with salt, pepper, smoked paprika, and garlic powder.
Heat the olive oil in a large skillet over medium-high heat. Sear the brisket for 2 to 3 minutes per side until nicely browned.
Place the sliced onion and garlic in the bottom of a 6-quart slow cooker, then set the brisket on top.
In a bowl, whisk together the soy sauce, honey, brown sugar, beef broth, ketchup, and apple cider vinegar. Pour the mixture over the brisket.
Cover and cook on low for 8 to 9 hours, or until the brisket is tender and easy to shred or slice.
Remove the brisket and let it rest for 10 minutes. Skim excess fat from the sauce if needed.
In a small bowl, mix the cornstarch and water. Stir it into the slow cooker juices, then cook on high for 10 to 15 minutes until the sauce thickens.
Slice the brisket against the grain and spoon the honey garlic sauce over the top. Sprinkle with parsley if you like and serve warm. Leftovers keep well in the fridge for up to 3 days.
