Mix salt, pepper, garlic powder, onion powder, and oregano in a small bowl.
Rub the spice mix all over the brisket.
Put the brisket in the slow cooker, fat side up.
Sprinkle minced garlic over it and add sliced onions.
In a bowl, mix beef broth, soy sauce, and ketchup. Pour it over the brisket until it's covered.
Cook on LOW for 8 hours.
Once done, keep it warm in the slow cooker or transfer it to a cutting board, cover with foil, and let it rest for 20 minutes.
Mix arrowroot powder and water in a small bowl to make a slurry.
Pour it into the liquid in the slow cooker and whisk. It'll thicken in 2-3 minutes.
Slice the meat and serve with the sauce