First of all. Cut the steak into 1 inch chunks and add to a rimmed baking pan along with the mushrooms.
And then, In a medium bowl, whisk together the oil, vinegar, garlic, salt, pepper, rosemary and oregano. Pour over the steak and mushrooms. Cover and refrigerate for at least 2 hours.
Step three, Meanwhile, boil the potatoes in a large pot of salted water until just tender, about 8-10 minutes. Drain and cool slightly.