Cook steak to your liking, remove from pan and rest. Add mushrooms, red onion and fry till soft. Add butter and melt down. Add flour and mix till all liquid thickens, stirring constantly. Add beef stock and continue stirring till a nice sauce develops. You will know what it should be like as soon as its ready. You may need more stock. This will depend on your own preference as to how thick you like the sauce. Remove from flame and stir in spinach leaves. Top steak with sauce.

Note in my picture I also had potato and garnished with oven roasted capsicum. Works well with roasted vegetables or a side of fries. Its really up to you.