Preheat the pan: Heat a large cast-iron pan over high heat (open a window—things will get smoky). Season the steak generously with salt and pepper. When the pan starts to smoke, add 15 ml of oil and swirl. Sear the steak, flipping every minute, for about 6 minutes total for medium-rare. Transfer to a plate.

Make the sauce: Remove the pan from heat. Pour in the wine, scraping up browned bits with a wooden spoon. Stir in the butter until melted. Season lightly with salt and pepper, then pour into a small serving dish and set aside.

Cook the mushrooms: Return the pan to medium-high. Add mushrooms and cook, stirring often, adding more oil if needed, until golden, 5–7 minutes. Spoon over the steak.

Wilt the spinach: Remove the pan from heat. Add remaining oil, garlic, and spinach. Stir constantly until just wilted, about 1 minute. Serve alongside the steak with the sauce on the side.