Preheat oven to 200°C. Grease and line a baking tray with baking paper.
Combine butter, parsley, chives, oregano, rosemary, garlic and mustard in a bowl. Season with salt and pepper. Place the butter mixture in the centre of a large piece of baking paper. Starting from 1 long edge, roll the butter into a sausage shape, twisting the ends of the paper to secure. Place in the fridge until firm.
Pat potato dry with paper towel. Place potato, garlic, rosemary and oil in a large bowl and toss to coat. Place on the prepared baking tray. Season with salt and pepper. Bake for 40 mins or until golden and crisp, turning once.
Meanwhile, spray a chargrill or barbecue plate with oil spray and heat over medium-high heat. Season steaks with salt and pepper and cook for 3-4 mins each side for medium rare or until cooked to your liking. Transfer to a plate and cover with foil. Set aside for 5 mins to rest.
Slice the butter into 1cm rounds. Divide steaks, potato and green beans among serving plates. Top the steak with the herb butter to serve.