Mash the onions into a puree using a hand blender. Mix with the mustard, garlic and soy sauce.
This amount of marinade is suitable for about 3.5 lb (1.5 kg) of meat. Place your seasoned steaks in a deep container that you can close.
Put them in the fridge for a few hours, while turning them over regularly during that time.
After they're done marinating, you can cook them however you like.