Heat skillet on medium high heat. Add oil then add the onions and garlic. Cook until softened. Add beef and cook until brown. Stir in tomatoes, chili powder, cumin, salt, and pepper. Remove from heat and drain any excess juices.
Arrange taco shells snugly standing up in an about 9x13" baking dish. Divide the meat mixture between all the taco shells.Sprinkle cheese over the tops of the filled tacos and bake for about 15 minutes, or until the cheese is melted and taco shells are crispy. Remove from oven.
Top with shredded lettuce, salsa, cilantro, guacamole, sour cream or your favorite taco toppings! Serve with lime wedges.