Fill a large pot with 1-inch water and fit with a steamer basket. Bring to a simmer. Place the cauliflower in the steamer basket, cover, and steam until very tender, 18 to 22 minutes. Transfer to a food processor, add the butter, ½ teaspoon salt, and ¼ teaspoon pepper and puree until smooth.
Meanwhile, heat the oil in a large skillet over medium-high heat. Season the steak with ¼ teaspoon each salt and pepper and cook, 3 to 5 minutes per side for medium-rare. Let rest for 5 minutes before slicing; reserve the skillet.
Add the mushrooms to the drippings in the skillet and cook, tossing occasionally, until browned and tender, 3 to 5 minutes. Add the wine and cook, stirring occasionally, until slightly reduced and syrupy, 2 to 3 minutes more; season with ¼ teaspoon each salt and pepper.
Serve the steak with the cauliflower puree. Top with the mushroom sauce and sprinkle with the parsley.