Pat the sirloin steak dry and season both sides evenly with chili powder, cumin, garlic powder, sea salt, and black pepper.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the hot skillet and sear for 3-4 minutes per side until a golden crust forms and it reaches your desired level of doneness.
Remove the steak from the pan and let it rest on a cutting board for 5 minutes before slicing against the grain into thin strips.
While the steak rests, lightly toast the corn tortillas in the same skillet or over an open gas flame until they are warm and pliable.
Thinly slice the radishes and avocado, and finely dice the red onion and cilantro.
Assemble the tacos by dividing the steak strips between the tortillas, then topping with avocado slices, radishes, onion, and cilantro.
Squeeze the fresh lime juice over the tacos just before serving for a bright, citrusy finish.