Season all sides of the beef with a fair amount of salt and pepper. In a large Dutch oven, or if your crock pot has a stove top insert, heat 2 tablespoons of olive oil over moderately high heat. Add the pot roast and brown. Get a nice brown crust on each side of the beef.
Place the beef in the crockpot. Sprinkle the garlic and onion over the beef.
Add the crushed tomatoes, 1 cup of water and add spices.
Cook in crock pot on low for 6 - 8 hours. Let meat cool in the liquid. Shred meat and set aside.
Warm corn tortillas by dampening a clean kitchen towel with water. Place the corn tortillas on the wet towel and fold the towel over the tortillas. Place the package in a gallon zip lock bag, but do not seal.
Place them in the microwave and cook for two minutes at 50% power. Let them steam in the bag for 4 minutes before removing.
Toppings for tacos: Shredded Cabbage, Avocado, guacamole, Sour Cream, sliced radishes, cilantro, salsa.