If you haven’t got two onions you can use two carrots or two potatoes. Place the brisket on top of the potatoes. Add the brisket … In a small bowl mix together the salt, pepper, garlic powder, onion powder, and oregano. How to cook beef brisket in the slow cooker. Slow Cooker Instructions . Cut two onions in half and add them to the bottom of the slow cooker. Sprinkle the minced garlic over the brisket … Use vegetables to stop the brisket from burning. Rub the spice mixture all over the brisket on both sides, then place the brisket in a slow cooker, fat side up.