In a bowl, combine the chili powder, brown sugar, Worcestershire sauce, and cumin together to make a paste. Rub the paste all over the brisket and place it in the slow cooker. Sprinkle brisket with desired amount of pepper followed by the onion.

In the same bowl you mixed the paste in, mix together the garlic, beef broth, liquid smoke, and chili sauce. Pour over the brisket.

Cover and cook on low for 7-8 hours.

Remove from slow cooker and let rest for 5 minutes.

Thicken cooking sauce in a sauce pan on the stove using about 1 Tbsp cornstarch with 1 1/2 Tbsp water. Cook until thick. You may need more cornstarch and water mixture if sauce is not thicken enough.

Slice brisket and serve with sauce. Serve 5-6 people.