Pat the sirloin steak dry with paper towels and season both sides with sea salt, black pepper, and garlic powder.
Heat the olive oil in a heavy skillet over medium-high heat until shimmering.
Place the steak in the hot pan and sear for 3-4 minutes per side for medium-rare, then remove from the pan and let it rest for 5 minutes.
While the steak rests, mash the avocado in a small bowl and whisk in the Greek yogurt, lime juice, and chopped cilantro until smooth.
Thinly slice the radishes and finely dice the red onion for the topping.
Warm the corn tortillas in a dry pan or over an open flame for 30 seconds per side until pliable.
Slice the rested steak against the grain into thin strips.
Assemble the tacos by dividing the steak between the tortillas, drizzling with avocado crema, and garnishing with radishes and red onion.