In a small bowl mix together the rosemary salt, black pepper, garlic powder and onion powder. Rub the spice mixture all over both sides of the brisket.
Heat the rosemary olive oil, over medium-high heat, in a large cast iron skillet or heavy bottomed pan. Sear the brisket for 4-5 minutes just until a nice crust forms. Turn and do the same to the other side. Remove from the skillet and transfer to a slow cooker, fat side up.
Sprinkle the minced garlic and sliced onion on and around the brisket.
In a medium bowl, combine the low sodium beef broth, soy sauce, maple balsamic vinegar, and ketchup. Pour the sauce over and around the brisket.
Cook on low for 8 hours. Once done cooking, transfer the brisket to a cutting board and cover with loose foil to let rest for 20-30 minutes.
While the brisket rests, combine the cornstarch and cold water in a small bowl to create a slurry. Pour the slurry into the liquid that is left in the slow cooker and whisk. Put the lid back on and the liquid will begin to thicken in 4-5 minutes.
Slice the brisket against the grain and place in a large shallow serving dish. Pour sauce on top and serve immediately.