Heat a large skillet on medium heat and add the olive oil.
Add the onions and garlic. Stir.
Add the ground beef, ground cumin, salt and pepper. Using a spatula or wooden spoon, stir while breaking the ground beef into little pieces that are all about the same size so that it cooks evenly.
Continue to cook until the meat is thoroughly cooked (about 7 to 10 minutes in total).
Turn the heat off and transfer the meat mixture on to a paper towel-lined plate to drain off any excess grease.
Add the meat mixture back into the pan. Heat the pan on low to medium heat.
Add the enchilada sauce and stir.
Continue to stir until the enchilada sauce is warm and it completely coats the ground beef (about 5 minutes).
Serve while warm in crispy taco shells. Top with shredded lettuce and cheddar cheese, and/or taco toppings of your choice.