Sprinkle steak with salt and pepper.
Broil until medium or medium-rare.
Slice steak thinly across the grain.
In a large skillet over medium heat, melt butter and cook until just beginning to brown, about 3 to 4 minutes.
Add mushrooms and sage and sauté about 4 minutes more.
Arrange spinach leaves on large plate and place sliced steak on top.
Pour mushroom sauce over each and sprinkle with salt and pepper, if desired.