How to make
Mash the onions into a puree using a hand blender. Mix with the mustard, garlic and soy sauce.
Gradually pour in the beer. Stir and use the resulting sauce to smear the tenderized steaks with.
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.
