Whisk together the olive oil, vinegar, scallions, capers, garlic, ½ teaspoon salt and ½ teaspoon pepper. Pour about ⅓ of the dressing (about ⅓ cup) over the steak and turn to coat both sides.
Add the parsley and 1 tablespoon mint to the reserved dressing, stir, and set aside until ready to use. Cover and refrigerate the steak for at least 30 minutes or up to 24 hours.
In a small sauté pan set over medium heat, toast the pistachio, tossing often, until golden brown, about 3 minutes. Set aside.
Set the grill to medium-high heat, or heat a grill pan on the stovetop over medium-high. Grill about 5 minutes per side Transfer to a plate to rest for 10 minutes
While the steak rests, cut the romaine hearts lengthwise into quarters. Arrange the romaine and radicchio in layers on a large platter, leaving room on one side for the steak. Sprinkle the goat cheese, pistachio and the remaining mint. Slice the steak across into 3-inch pieces, then slice against the grain to cut the steak into wide strips. Add sliced steak to the platter, drizzle the reserved dressing on top of both. Serve immediately.