In a blender combine wine, onion, lemon juice, oil, garlic, salt, and pepper.
Whirl until blended.
Pour over steak, cover, and refrigerate 1 to 2 hours.
Remove steak from marinade and drain briefly (reserve marinade); set steak aside.
In a wide frying pan, melt 4 tablespoons of the butter over medium heat.
Add mushrooms and cook for about 5 minutes.
Add reserved marinade and turn the heat to high.
Boil mixture, stirring constantly, until just slightly reduced and thickened.
Keep warm while steak cooks.
Place steak on lightly greased grill 4 to 6 inches above a solid bed of glowing coals.
Cook, turning once, for about 8 minutes on each side for rare or until done to your liking when slashed.
When you turn steak, place bread, butter-side-down, around outside of grill to toast.