In a shallow dish, whisk together 1 tablespoon of lime juice, olive oil, cumin, garlic powder, sea salt, and black pepper.
Add the flank steak to the marinade, coating both sides thoroughly, and let it sit for at least 15 minutes at room temperature.
Heat a cast-iron skillet or grill over medium-high heat and sear the steak for 4-5 minutes per side until it reaches your desired level of doneness.
Remove the steak from the heat and let it rest on a cutting board for 5 minutes before slicing thinly against the grain.
While the steak rests, prepare the crema by mashing the avocado with the Greek yogurt and 1 teaspoon of lime juice until smooth and creamy.
Warm the corn tortillas in a dry pan over medium heat for 30 seconds per side until pliable.
Assemble the tacos by dividing the sliced steak between the tortillas, then top with avocado crema, fresh cilantro, and diced red onion.