How to Make It
Preheat broiler with oven rack in top position. Place a rimmed baking sheet in oven (do not remove pan while oven preheats).
Sprinkle steak with 1/2 teaspoon salt and 1/2 teaspoon black pepper. Carefully remove pan from oven; add 1 tablespoon oil to pan, tilting pan to coat. Add steak to pan; broil 5 minutes. Remove pan from oven; turn steak over. Add 1 tablespoon oil to pan around steak. Arrange broccoli and onion evenly around steak; sprinkle vegetables with 1/4 teaspoon salt and remaining 1/4 teaspoon black pepper. Return pan to oven; broil until a thermometer inserted in steak registers 140°F, about 5 minutes. Place steak on a cutting board. Continue to broil vegetables until tender and lightly charred, about 5 minutes.
Whisk together remaining 1/4 teaspoon salt, remaining 1/4 cup oil, parsley, vinegar, oregano, capers, and red pepper in a bowl. Cut steak against the grain into thin slices. Spoon parsley mixture evenly over steak. Serve with vegetable mixture.