Heat the broiler to high with the rack as close to the heat source as possible, or heat a gas grill, covered, to high.
Put the garlic and ginger in a food processor and pulse until roughly chopped. Scrape the sides with a spatula, add the scallions, 3 Tbs. of the peanut oil, 2 Tbs. of the soy sauce, 1 tsp. of the sesame oil, and process until the mixture has a paste-like texture and the garlic and ginger are minced. Season well with pepper. Use the thin tip of a sharp knife to score both sides of the meat in a diagonal crosshatch pattern, going across the grain, making each slit about 1/4-inch deep. Put the meat in a 9x13-inch nonreactive dish and season well with salt. Pour the marinade over the meat and rub it into the scored crevices.
Lay the meat either on a cold broiler pan or on the hot grill; broil or grill, turning once, until done to your liking, 8 to 10 minutes. Using tongs, transfer the meat to a warmed serving tray and let it rest for 5 minutes. In a small bowl, whisk together the remaining 3 Tbs. peanut oil, 1-1/2 Tbs. soy sauce, 3/4 tsp. sesame oil, and the rice vinegar. Slice the meat thinly on the diagonal and serve drizzled with the sauce.
You can marinate the steak a day ahead, refrigerated, but salt it just before cooking.