In a small bowl mix together the salt, pepper, garlic powder, onion powder, and oregano. Rub the spice mixture all over the brisket on both sides, then place the brisket in a slow cooker, fat side up. Sprinkle the minced garlic over the brisket and top with the sliced onions, potatoes and carrots.
In a medium bowl whisk together the broth, soy sauce, and ketchup. Pour into slow cooker until the brisket is just covered.
Cook on LOW for 7-8 hours or on HIGH for 4-6 hours. Once done cooking, either keep in the slow cooker on WARM, or transfer to a wood board and cover with foil and let rest for 20 minutes.
Meanwhile, whisk together the arrowroot powder and water in a small bowl to create a slurry. Pour the slurry into the liquid in the slow cooker and whisk. The liquid will begin to thicken after 2-3 minutes.
Slice the meat against the grain and serve with sauce and veggies on top.
