In a small bowl, combine kosher salt, black pepper, smoked paprika, garlic powder, onion powder, and chili powder to create a flavorful spice rub. Generously coat the brisket with this mixture.

Heat about 2 tablespoons of olive oil in a large skillet over medium-high heat. Sear the brisket for 4-5 minutes on each side until a dark, caramelized crust forms.

While the brisket is searing, slice the onions and scatter them along with the minced garlic at the bottom of your slow cooker. Once the brisket is perfectly seared, transfer it to the slow cooker.

In a medium bowl, whisk together low-sodium beef broth, Worcestershire sauce, soy sauce, brown sugar, Dijon mustard, and apple cider vinegar until well combined. Pour this mixture over the brisket in the slow cooker.

Cover the slow cooker and set it to low for 8-10 hours or high for 4-5 hours. The brisket should be tender and easily shredded with a fork.

Once cooked, remove the brisket from the slow cooker and tent it loosely with foil. Slice the brisket against the grain and serve it drizzled with the sauce.