Adjust oven rack to highest position and heat oven to 500 degrees. Spray a large (18- by 12-inch) rimmed baking sheet with vegetable cooking spray and place the 4 cauliflower steaks on it. Brush both sides of the cauliflower steaks with the oil and sprinkle both sides generously with salt and pepper. Place in the oven and roast until cauliflower is cooked through and impressively colored, about 25 minutes.