Preheat oven to 450°. Set aside 2 tablespoons of butter in a small bowl to soften at room temperature.
Slice the potatoes into 1” cubes and cut broccoli into florettes, bite-size pieces. Toss potatoes with 1Tbsp of olive oil, salt and pepper and spread out on a baking sheet. Toss broccoli with 1 Tbsp of olive oil, salt and pepper, and a 1/4 tsp of red pepper flakes. Roast in the oven until broccoli is slightly browned and potatoes are tender, around 15 to 20 minutes.
While the vegetables are roasting, finely chop the parsley. Place softened butter into a small bowl and add parsley, mustard, and a pinch of minced garlic (to taste) season with salt and pepper. Mash with a fork to combine.
Pat steaks dry with a paper towel and season with salt and pepper. Heat 1 tablespoon of olive oil in a large pan over medium high heat. Cook to desired doneness, 3 to 5 minutes per side should do. Set aside and rest 5 minutes.
After the broccoli has browned, remove from the baking sheet. Toss the potatoes and sprinkle with 2 tablespoons of Parmesan. Roast until tender, about five more minutes.
Thinly slice steak and plate with potatoes and broccoli. Top steak with the butter mixture and garnish with parsley and Parmesan if desired.