Thinly slice red onion, place in a small bowl, pour vinegar and juice of 1 lime over it, add a pinch of salt, and let sit.
Brown ground beef in a large skillet over medium-high heat, breaking it apart with a spoon, about 5β6 minutes; drain excess fat.
Stir taco seasoning and 1/4 cup water into the beef and simmer 2 minutes until slightly thickened.
Warm tortillas in a dry skillet or over a flame for 10 seconds per side.
Assemble tacos with beef, pickled onions, cilantro, and a squeeze of fresh lime juice.