In a heat proof bowl, combine the onion with the ingredients through the cumin and microwave until the onions are softened, about 5 minutes, stirring occasionally.
Stir the tomato sauce, sugar, and liquid smoke into the slow cooker with the onion mixture.
Season the beef with salt and pepper on both sides. Place in slow cooker and spoon sauce over top. Cover and cook on low for 9 to 11 hours.
Transfer the beef to a bowl and discard any excess fat. Shred the beef with two forks.
Skim any fat from the surface of the liquid remaining in the slow cooker. Ladle 1 cup, or more if needed, into the shredded beef to moisten and add flavor.
Kickin' Burritos
Lay the tortilla on a flat surface.
Spread an even layer of guacamole, salsa, cheese, black beans, rice, and a large portion of shredded beef.
Fold the ends up and wrap.
If you are feeling feisty, toast the burrito in a preheated skillet over medium heat with a weight (like a plate) pressing down on top until golden. Flip and repeat.
Serve with sour cream and hot sauce, if desired. Live Mas.