Preheat oven to 425°F.
Soften the potatoes: Clean and poke holes in the potatoes with a fork. Microwave for 3–6 minutes to soften slightly (time will vary based on size—larger potatoes need closer to 5–6 minutes).
Dice the potatoes into bite-sized cubes. Smaller pieces will help ensure they cook evenly with the steak.
Combine everything: In a large bowl, toss the steak, potatoes, green beans, mushrooms, and onions with the herbs and olive oil (or melted butter), until everything is well coated.
Transfer to a baking dish: Spread the mixture in a 9x13-inch casserole dish. Cover tightly with foil.
Bake for 30–40 minutes, or until steak reaches your preferred doneness and potatoes are tender. For medium steak, start checking around the 30-minute mark. For rare, reduce the time slightly. For well-done, bake a bit longer.
Serve hot, garnished with fresh parsley or a sprinkle of Parmesan if desired.