Heat a cast-iron skillet over medium-high heat. Add 1/4 of the oil. Dry the steak well and season with salt and pepper. Place the steak in the pan and cook for about 10 to 15 minutes, flipping once. The timing will depend on how thick your steak is and your desired doneness. Remove, let rest for 10 minutes, and then slice against the grain and set aside.
Meanwhile, in a small bowl or jar, add the remaining oil, lime juice, sriracha, and season with salt and pepper. Stir or shake to combine.
Place the lettuce, onion, and radishes into a bowl or plate and top with the sliced steak. Drizzle the dressing on top and garnish with cilantro. Enjoy!
Leftovers: Salad is best stored separately from the dressing. Refrigerate in an airtight container for up to three days.