Pat the beef brisket dry with paper towels and trim any very thick or hard pieces of fat, leaving a thin layer for flavor.

In a small bowl, combine the kosher salt, black pepper, smoked paprika, garlic powder, onion powder, dried thyme, and dried oregano, stirring until evenly mixed.

Rub the spice mixture all over the brisket, coating both sides and pressing it gently into the meat so it adheres well.

Heat the olive oil in a large skillet over medium-high heat until shimmering.

Sear the brisket in the hot skillet for 3 to 4 minutes per side, until a deep golden-brown crust forms, then transfer the brisket to a plate and set aside.

Place the sliced yellow onions in an even layer on the bottom of the slow cooker, then sprinkle the minced garlic over the onions.

In a medium bowl, whisk together the beef broth, tomato sauce, Worcestershire sauce, brown sugar, and apple cider vinegar until the brown sugar is mostly dissolved.

Pour the broth mixture evenly over the onions and garlic in the slow cooker.

Nestle the seared brisket on top of the onions in the slow cooker, fat side up, so the juices baste the meat as it cooks.

Cover the slow cooker with its lid and cook the brisket on LOW for 8 hours, or until the meat is very tender and easily pulls apart with a fork.

When the brisket is done, carefully transfer it to a cutting board and let it rest for about 10 to 15 minutes before slicing.

While the brisket rests, ladle the cooking liquid and onions from the slow cooker into a saucepan and bring to a gentle simmer over medium heat.

In a small bowl, stir together the cornstarch and cold water until completely smooth to create a slurry.

Slowly whisk the cornstarch slurry into the simmering sauce, cooking for 2 to 3 minutes until the sauce thickens to a gravy-like consistency, then reduce the heat to low.

Slice the rested brisket against the grain into thin slices, returning the slices to the slow cooker or placing them on a serving platter.

Spoon the thickened onion sauce over the sliced brisket and sprinkle with the chopped fresh parsley before serving.