In a small bowl, combine the lime juice, chili powder, ¼ teaspoon of salt, and garlic to form a paste. Rub onto both sides of the steak, then transfer to a plate and refrigerate.
Place the potatoes in a large pot covered with cold water. Bring to a boil over high heat, and cook until tender when pierced with a fork, 15-20 minutes, depending on how large your potatoes are. Drain and allow to cool for 10 minutes. Slice into quarters.
Preheat a grill pan (or grill) over medium-high heat. Oil the pan, then grill the steak, flipping once, for 8-10 minutes on each side (if using a grill, you’ll only need about 5 minutes per side), until a meat thermometer inserted into the thickest part of the steak reads 140˚F. Transfer to a cutting board and let rest for 5 minutes before slicing into ¼-inch slices.
In a large bowl, whisk together the vinegar, oil, cumin, black pepper, and remaining ¼ teaspoon of salt. Add the steak and any juices, the potatoes, radishes, scallions, and cilantro. Gently toss to combine.