Combine the first five ingredients in a small bowl; season with a generous pinch of salt. Pour half of the marinade into a small container and refrigerate to use later on the kale. Stir the ginger and garlic into the remaining marinade. Place the steak in a small, deep container; pour the marinade over the steak, turning it to coat. Dover and refrigerate at least 3 hours, or up to 24 hours.
Heat a grill pan over medium-high heat. Add the scallions and cook 4-5 minutes, or until the scallions soften and start to lightly char. Remove the scallions from the heat and slice them thinly.
Add 1 tablespoon of oil to the grill pan. When the oil starts to shimmer, add the steak (discard the marinade). Grill for 15 minutes, turning once, or until the steak reaches an internal temperature of 140ºF. Remove from the heat and let rest 10 minutes before slicing thinly against the grain.
Remove the tough stems and ribs from the kale, then thinly slice the leaves. In a large bowl, combine the kale, reserved marinade, and lime juice. Use your hands to massage the dressing into the kale until it begins to soften. Season to taste with salt and pepper. Toss in the mint, basil, sliced scallions, radishes, and cashews.
Top the salad with the sliced steak. Serve with lime wedges, if desired.