For sauce, in a blender container or food processor bowl combine parsley, olive oil, lemon juice, capers, salt, hot pepper sauce, and garlic. Cover and blend or process until nearly smooth, stopping and scraping sides as necessary. Set aside.
Preheat broiler. Trim fat from meat. Place meat on the unheated rack of a broiler pan. Broil 3 to 4 inches from the heat for 15 to 17 minutes for medium-rare (145 degree F) and 20 to 22 minutes for medium (160 degree F), turning once halfway through broiling.
Thinly slice meat. To serve, arrange onion rings and mesclun on toasted bread slices; top with meat slices. Drizzle with sauce. Makes 4 servings.