01 - Combine apple cider vinegar, water, sugar, and salt in a small saucepan. Bring to a boil, stirring to dissolve sugar and salt. Pour the hot mixture over thinly sliced red onions placed in a heatproof jar or bowl. Let sit at room temperature for at least 20 minutes.
02 - Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook for 2 to 3 minutes until softened. Stir in minced garlic and cook for an additional 30 seconds.
03 - Add ground beef to skillet, breaking it up with a spoon. Cook until browned, about 5 to 7 minutes, draining excess fat if necessary.
04 - Stir in ground cumin, smoked paprika, chili powder, dried oregano, salt, and black pepper. Pour in beef broth or water and simmer for 3 to 5 minutes until most liquid evaporates and flavors meld.
05 - Heat tortillas in a dry skillet or microwave until pliable and warm.
06 - Fill each tortilla with the beef mixture, then top with shredded lettuce, diced tomatoes, drained pickled onions, crumbled cheese, and fresh cilantro. Serve with lime wedges.