Preheat oven to 400°F. On a baking sheet lightly coated with cooking spray, arrange onion on 1 end. Drizzle the 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 and sprinkle with salt and pepper. Arrange the steak, cauliflower, and broccoli in a single layer on baking the sheet with the onion. Bake at 400°F for 8 minutes, then set it aside for a moment.

Arrange the top oven rack 5 inches from broiler element, and preheat the broiler to high.

Toss beans with 1 teaspoon of the oil, then sprinkle with salt and pepper. Arrange beans on the baking sheet with steak and vegetables. Broil everything for 7 minutes or until steak and vegetables are charred. A meat thermometer in the steak should register at 135°F (for medium rare) or whatever degree of doneness you prefer. Remove steak from baking sheet; let stand at room temperature 6 to 8 minutes before slicing. Thinly slice across the grain.

Lastly, stir together butter, parsley, shallot, lemon zest, salt, garlic, and remaining 1 teaspoon oil. Place butter mixture on the hot steak to melt as it rests, brushing it to coat the steak evenly. Serve with vegetables.