In a small bowl, combine the salt, pepper, garlic powder, onion powder, and oregano. Rub this spice mixture evenly over all sides of the brisket.
Spread the sliced onion on the bottom of your slow cooker. Place the brisket on top, fat-side up. Scatter the minced garlic, baby potatoes, and carrot chunks around the meat.
In a separate bowl, whisk together the beef broth, soy sauce, and ketchup until smooth. Pour this liquid into the slow cooker.
Cover and cook on LOW for 8 to 10 hours or on HIGH for 4 to 5 hours, until the brisket is very tender.
Carefully transfer the brisket and vegetables to a cutting board or platter. Cover loosely with foil and let the brisket rest for at least 15 minutes.
While the meat rests, prepare the sauce. In a small bowl, whisk the cornstarch and cold water together to form a slurry.
Pour the slurry into the liquid remaining in the slow cooker. Whisk to combine. Set the slow cooker to HIGH, cover, and cook for 15-20 minutes, or until the sauce has thickened.
Check that the brisket's internal temperature has reached a minimum of 145°F (63°C). For maximum tenderness, the temperature is often closer to 195°F (90°C).
Slice the brisket against the grain into 1/4-inch (6 mm) thick pieces. Serve immediately with the vegetables and the thickened sauce.