Lay onions in a 6-Quart Crock Pot
Lay chuck roast over onions
Combine the beef broth, tomato paste and seasonings in a measuring cup.
Pour broth mixture over chuck roast and cook on high for 4 hours or low for 8 hours.
Once meat is cooke, shred with two forks in the tomato and broth mixture.
Put beef on tortillas and top with toppings of your choice.