Heat a large skillet (preferably cast iron) on medium high heat. Add oil.
While the pan is heating, rub brisket with garlic powder on both sides. Quickly brown the brisket on both sides in the skillet, about 3-4 minutes a side.
While brisket is browning, layer onions and mushrooms on the bottom of a 5-6 quart slow cooker.
Move brisket to slow cooker on top of the onions.
Pour the broth and liquid smoke into the skillet and stir with a wooden spoon to loosen any browned bits. Pour the liquid into the slow cooker on the side of the meat, not on top of it. You want the juices and onions on the bottom and the meat on top out of the liquid.
Cook 3-4 hours on high, or 5-6 on low, depending on your slow cooker.
Remove brisket to cutting board and tent with foil.
Pour juices into a small saucepan and whisk in cornstarch. Bring to a boil and cook for 1 minute. Add cooked onions and mushrooms.
Slice brisket against the grain. Serve with gravy.