In a small bowl, whisk together the olive oil, lime juice, minced garlic, cumin, chili powder, sea salt, and black pepper to create the marinade.
Place the flank steak in a shallow dish and coat thoroughly with the marinade, allowing it to sit for at least 20 minutes at room temperature.
Heat a cast-iron skillet over high heat until very hot, then sear the steak for 4-5 minutes per side until a deep brown crust forms.
Remove the steak from the skillet and let it rest on a cutting board for 5 minutes to lock in the juices.
While the meat rests, dice the white onion and finely chop the fresh cilantro.
Briefly toast the corn tortillas in the hot skillet for 30 seconds per side until pliable and slightly charred.
Slice the steak against the grain into thin strips and then into bite-sized pieces.
Assemble the tacos by dividing the steak between the tortillas, topped with the onion, cilantro, and sliced avocado.
Serve immediately with wedges cut from the fresh lime for squeezing over the top.