In a small bowl, whisk together the lime juice, minced garlic, cumin, chili powder, sea salt, and black pepper.
Place the flank steak in a shallow dish and coat thoroughly with the marinade; let it sit for at least 20 minutes.
While the steak marinates, mash the avocado with the Greek yogurt and a squeeze of lime until completely smooth to create the crema.
Heat the avocado oil in a cast-iron skillet over medium-high heat until the pan is shimmering and very hot.
Sear the flank steak for 3-4 minutes per side for medium-rare, ensuring a dark, caramelized crust forms on both sides.
Remove the steak from the heat and let it rest on a cutting board for 5 minutes before slicing thinly against the grain.
Warm the corn tortillas in a dry pan for 30 seconds per side until pliable.
Assemble the tacos by layering the sliced beef into the tortillas, followed by a dollop of avocado crema, fresh cilantro, and diced red onion.