How to Make It
Preheat oven to 400°F. Arrange onion on 1 end of a baking sheet lightly coated with cooking spray; drizzle onion with 1 teaspoon of the oil, and sprinkle with salt and pepper. Bake in preheated oven for 5 minutes.
Sprinkle steak with salt and pepper, and drizzle with 1 tablespoon of the oil. Toss cauliflower and broccoli with 1 tablespoon of the oil; sprinkle with salt and pepper. Arrange steak, cauliflower, and broccoli in a single layer on baking sheet with onion. Bake at 400°F for 8 minutes. Remove from oven.
Arrange top oven rack 5 inches from broiler element. Preheat broiler to high.
Toss beans with 1 teaspoon of the oil; sprinkle with salt and pepper. Arrange beans on baking sheet with steak and vegetables. Broil 7 minutes or until steak and vegetables are charred and a meat thermometer registers the steak at 135°F (for medium rare) or until desired degree of doneness. Remove steak from baking sheet; let stand at room temperature 6 to 8 minutes before slicing. Thinly slice across the grain.
Meanwhile, stir together butter, parsley, shallot, lemon zest, salt, garlic, and remaining 1 teaspoon oil. Place butter mixture on hot steak to melt as it rests; brush evenly over steak. Serve with vegetables.