Cheddar or Monterey Jack Cheese - 4 cups, shredded
Cover the bottom of the slow cooker pot with sliced onions. Add beef roast and water. Sprinkle with taco seasoning (1 package).
Set the slow cooker on high and cook, covered, for 5 hours. Transfer beef to a platter/large bowl and shred with two forks. Put it back into the slow cooker; mix in the other package of taco seasoning.
Set the slow cooker on high and cook, covered, for 30 minutes in order to bring the beef to boiling.
Follow the package instructions to heat the tortillas. Divide the beef using a draining spoon between the tortillas. Sprinkle each with cheese; add any topping (diced tomatoes, black beans, diced onions, sour cream, sliced jalapeno). Serve with Pico de Gallo.