Season the beef brisket on all sides with salt, pepper, garlic powder, onion powder, and dried oregano.

Place the sliced onion at the bottom of the slow cooker and lay the seasoned brisket on top.

In a bowl, whisk together the beef broth, soy sauce, and ketchup. Pour this mixture over the brisket.

Cover the slow cooker and cook on low for 8 hours, or until the brisket is tender and easily pulls apart.

Once cooked, remove the brisket from the slow cooker and let it rest for a few minutes before slicing.

To make the gravy, mix cornstarch with cold water until smooth. Pour into the slow cooker and stir to combine with the cooking juices. Cook on high for an additional 10-15 minutes until the gravy thickens.

Slice the brisket against the grain and serve with the thickened gravy.