Season flank steak on both sides with kosher salt and freshly ground black pepper. In a bowl, whisk olive oil, Worcestershire, fish sauce, lime juice, green onions, shallots, cilantro, mint, sugar, and crushed red pepper. Place steak in a gallon-size resealable plastic bag with half the marinade and seal bag. Marinate in refrigerator for at least 1 hour and up to 4 hours.
Preheat a grill or broiler to medium-high.
Remove steak from marinade; discard marinade. Grill or broil steak for 3 to 4 minutes on each side for medium-rare. Remove from grill and let rest 10 minutes. Thinly slice steak against the grain, reserving any accumulated juices.
In another mixing bowl, combine cabbages, carrots, and cucumber with remaining marinade. Toss well and arrange on a platter. Arrange steak slices on top of cabbage. Garnish with crushed peanuts and serve immediately.