Preheat broiler. Pat steak dry. Stir together 1 teaspoon salt, cumin, coriander, and pepper in a small bowl and rub mixture onto both sides of steak. Broil steak on a broiler pan about 4 inches from heat 6 minutes per side for medium-rare. Transfer to a cutting board and let stand 5 minutes. Meanwhile, with motor running, add garlic to a food processor and finely chop. Add cilantro, parsley, vinegar, oil, cayenne, and remaining 1/2 teaspoon salt, then pulse until herbs are finely chopped. Holding a knife at a 45-degree angle, thinly slice steak. Serve with sauce.