1/4 cup finely chopped English hothouse or Persian cucumber
1/4 cup thinly sliced scallions
1 1/2 tablespoons fresh lime juice
1 teaspoon finely grated peeled ginger
8-ounce flank steak
1/2 teaspoon salt
1/4 teaspoon chili powder
16 small crunchy inner leaves of butter lettuce, romaine, or endive
2 tablespoons chopped salted roasted peanuts
2 tablespoons thinly sliced fresh mint leaves
Asian sweet chili sauce
Combine first four ingredients in a medium bowl. Season to taste with kosher salt and set aside.
Heat a grill pan or a lightly oiled cast-iron skillet over high heat. Season steak with salt and chili powder. Grill steak, turning once, until charred on both sides, about 8 minutes for medium-rare. Transfer to a cutting board and let rest for 10 minutes.
Arrange butter lettuce, romaine, or endive on a large platter. Thinly slice steak against the grain, then cut slices crosswise into 1-inch pieces. Add steak to cucumber mixture and toss to mix. Season to taste with salt and more lime juice, if desired.
Divide steak salad mixture among lettuce cups; garnish with peanuts and mint leaves. Drizzle with Asian sweet chili sauce.