Slice the flank steak into thin strips, then cut into small bite-sized pieces.
Heat vegetable oil in a large skillet over medium-high heat until hot.
Add the sliced beef to the skillet and sear without crowding the pan. Cook until browned, about 5 minutes.
Add half the chopped onion and cook until softened, about 3-4 minutes more.
Season with salt and pepper, then squeeze the juice from 2 limes over the meat mixture.
In a separate skillet, melt butter and lightly fry each corn tortilla for 30 seconds per side.
Assemble tacos using 2 tortillas per taco. Fill with meat mixture and top with remaining onion, cilantro, and lime wedges.