Heat a charcoal or gas grill until hot (you should barely be able to hold your hand 3 or 4 inches from the fire), or use a grill pan or medium-sized skillet on medium-high heat. Meanwhile, bring the steak to room temperature, pat it dry with paper towels, and sprinkle both sides with salt and pepper.
Grill or broil steak until nicely browned, 3 to 4 minutes per side. Move the meat to a cooler part of the grill and cook another 5 to 7 minutes, turning once
While the steak cooks, put the olive oil in a 12-inch skillet over medium-high heat. When its hot, add the greens, and sprinkle with tsp. each salt and freshly ground pepper. Cook, tossing frequently.
Cover steak and let rest for 5 minutes before cutting across the grain into thin slices. Serve with greens.