When making protein-rich foods, consider the whole package. Sirloin, for example, has about half the saturated fat on a porterhouse steak.
Preheat oven to 400 degrees. Make dressing: Whisk together lime juice, fish sauce, chile, and sugar in a small bowl until sugar is dissolved. Stir in cilantro and mint. Set aside. Toss together scallions, shallots, cucumbers, and arugula in a large bowl; set aside.
Heat oil in a large ovenproof skillet over high heat until hot but not smoking. Sprinkle steak all over with salt; season with pepper. Brown steak, turning once, about 2 minutes per side. Transfer skillet to oven; cook steak to desired doneness, about 6 minutes for medium-rare.
Transfer steak to a cutting board, and let stand at least 5 minutes. Cut steak across the grain into 1/4-inch-thick slices.
Add steak to salad. Drizzle with dressing, and toss. Sprinkle with peanuts.